feat: per-channel hashcash proof-of-work for PRIVMSG anti-spam (#79)

closes #12

## Summary

Implements per-channel hashcash proof-of-work requirement for PRIVMSG as an anti-spam mechanism. Channel operators set a difficulty level via `MODE +H <bits>`, and clients must compute a proof-of-work stamp bound to the channel name and message body before sending.

## Changes

### Database
- Added `hashcash_bits` column to `channels` table (default 0 = no requirement)
- Added `spent_hashcash` table with `stamp_hash` unique key and `created_at` for TTL pruning
- New queries: `GetChannelHashcashBits`, `SetChannelHashcashBits`, `RecordSpentHashcash`, `IsHashcashSpent`, `PruneSpentHashcash`

### Hashcash Validation (`internal/hashcash/channel.go`)
- `ChannelValidator` type for per-channel stamp validation
- `BodyHash()` computes hex-encoded SHA-256 of message body
- `StampHash()` computes deterministic hash of stamp for spent-token key
- `MintChannelStamp()` generates valid stamps (for clients)
- Stamp format: `1:bits:YYMMDD:channel:bodyhash:counter`
- Validates: version, difficulty, date freshness (48h), channel binding, body hash binding, proof-of-work

### Handler Changes (`internal/handlers/api.go`)
- `validateChannelHashcash()` + `verifyChannelStamp()` — checks hashcash on PRIVMSG to protected channels
- `extractHashcashFromMeta()` — parses hashcash stamp from meta JSON
- `applyChannelMode()` / `setHashcashMode()` / `clearHashcashMode()` — MODE +H/-H support
- `queryChannelMode()` — shows +nH in mode query when hashcash is set
- Meta field now passed through the full dispatch chain (dispatchCommand → handlePrivmsg → handleChannelMsg → sendChannelMsg → fanOut → InsertMessage)
- ISUPPORT updated: `CHANMODES=,H,,imnst` (H in type B = parameter when set)

### Replay Prevention
- Spent stamps persisted to SQLite `spent_hashcash` table
- 1-year TTL (per issue requirements)
- Automatic pruning in cleanup loop

### Client Support (`internal/cli/api/hashcash.go`)
- `MintChannelHashcash(bits, channel, body)` — computes stamps for channel messages

### Tests
- **12 unit tests** in `internal/hashcash/channel_test.go`: happy path, wrong channel, wrong body hash, insufficient bits, zero bits skip, bad format, bad version, expired stamp, missing body hash, body hash determinism, stamp hash, mint+validate round-trip
- **10 integration tests** in `internal/handlers/api_test.go`: set mode, query mode, clear mode, reject no stamp, accept valid stamp, reject replayed stamp, no requirement works, invalid bits range, missing bits arg

### README
- Added `+H` to channel modes table
- Added "Per-Channel Hashcash (Anti-Spam)" section with full documentation
- Updated `meta` field description to mention hashcash

## How It Works

1. Channel operator sets requirement: `MODE #general +H 20` (20 bits)
2. Client mints stamp: computes SHA-256 hashcash bound to `#general` + SHA-256(body)
3. Client sends PRIVMSG with `meta.hashcash` field containing the stamp
4. Server validates stamp, checks spent cache, records as spent, relays message
5. Replayed stamps are rejected for 1 year

## Docker Build

`docker build .` passes clean (formatting, linting, all tests).

Co-authored-by: user <user@Mac.lan guest wan>
Co-authored-by: Jeffrey Paul <sneak@noreply.example.org>
Reviewed-on: #79
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/hashcash/channel_test.go

@@ -0,0 +1,244 @@
+package hashcash_test
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"testing"
+
+	"git.eeqj.de/sneak/neoirc/internal/hashcash"
+)
+
+const (
+	testChannel  = "#general"
+	testBodyText = `["hello world"]`
+)
+
+func testBodyHash() string {
+	hash := sha256.Sum256([]byte(testBodyText))
+
+	return hex.EncodeToString(hash[:])
+}
+
+func TestChannelValidateHappyPath(t *testing.T) {
+	t.Parallel()
+
+	validator := hashcash.NewChannelValidator()
+	bodyHash := testBodyHash()
+
+	stamp := hashcash.MintChannelStamp(
+		testBits, testChannel, bodyHash,
+	)
+
+	err := validator.ValidateStamp(
+		stamp, testBits, testChannel, bodyHash,
+	)
+	if err != nil {
+		t.Fatalf("valid channel stamp rejected: %v", err)
+	}
+}
+
+func TestChannelValidateWrongChannel(t *testing.T) {
+	t.Parallel()
+
+	validator := hashcash.NewChannelValidator()
+	bodyHash := testBodyHash()
+
+	stamp := hashcash.MintChannelStamp(
+		testBits, testChannel, bodyHash,
+	)
+
+	err := validator.ValidateStamp(
+		stamp, testBits, "#other", bodyHash,
+	)
+	if err == nil {
+		t.Fatal("expected channel mismatch error")
+	}
+}
+
+func TestChannelValidateWrongBodyHash(t *testing.T) {
+	t.Parallel()
+
+	validator := hashcash.NewChannelValidator()
+	bodyHash := testBodyHash()
+
+	stamp := hashcash.MintChannelStamp(
+		testBits, testChannel, bodyHash,
+	)
+
+	wrongHash := sha256.Sum256([]byte("different body"))
+	wrongBodyHash := hex.EncodeToString(wrongHash[:])
+
+	err := validator.ValidateStamp(
+		stamp, testBits, testChannel, wrongBodyHash,
+	)
+	if err == nil {
+		t.Fatal("expected body hash mismatch error")
+	}
+}
+
+func TestChannelValidateInsufficientBits(t *testing.T) {
+	t.Parallel()
+
+	validator := hashcash.NewChannelValidator()
+	bodyHash := testBodyHash()
+
+	// Mint with 2 bits but require 4.
+	stamp := hashcash.MintChannelStamp(
+		testBits, testChannel, bodyHash,
+	)
+
+	err := validator.ValidateStamp(
+		stamp, 4, testChannel, bodyHash,
+	)
+	if err == nil {
+		t.Fatal("expected insufficient bits error")
+	}
+}
+
+func TestChannelValidateZeroBitsSkips(t *testing.T) {
+	t.Parallel()
+
+	validator := hashcash.NewChannelValidator()
+
+	err := validator.ValidateStamp(
+		"garbage", 0, "#ch", "abc",
+	)
+	if err != nil {
+		t.Fatalf("zero bits should skip: %v", err)
+	}
+}
+
+func TestChannelValidateBadFormat(t *testing.T) {
+	t.Parallel()
+
+	validator := hashcash.NewChannelValidator()
+
+	err := validator.ValidateStamp(
+		"not:valid", testBits, testChannel, "abc",
+	)
+	if err == nil {
+		t.Fatal("expected bad format error")
+	}
+}
+
+func TestChannelValidateBadVersion(t *testing.T) {
+	t.Parallel()
+
+	validator := hashcash.NewChannelValidator()
+	bodyHash := testBodyHash()
+
+	stamp := "2:2:260317:#general:" + bodyHash + ":counter"
+
+	err := validator.ValidateStamp(
+		stamp, testBits, testChannel, bodyHash,
+	)
+	if err == nil {
+		t.Fatal("expected bad version error")
+	}
+}
+
+func TestChannelValidateExpiredStamp(t *testing.T) {
+	t.Parallel()
+
+	validator := hashcash.NewChannelValidator()
+	bodyHash := testBodyHash()
+
+	// Mint with a very old date by manually constructing.
+	stamp := mintStampWithDate(
+		t, testBits, testChannel, "200101",
+	)
+
+	err := validator.ValidateStamp(
+		stamp, testBits, testChannel, bodyHash,
+	)
+	if err == nil {
+		t.Fatal("expected expired stamp error")
+	}
+}
+
+func TestChannelValidateMissingBodyHash(t *testing.T) {
+	t.Parallel()
+
+	validator := hashcash.NewChannelValidator()
+	bodyHash := testBodyHash()
+
+	// Construct a stamp with empty body hash field.
+	stamp := mintStampWithDate(
+		t, testBits, testChannel, todayDate(),
+	)
+
+	// This uses the session-style stamp which has empty
+	// ext field — body hash is missing.
+	err := validator.ValidateStamp(
+		stamp, testBits, testChannel, bodyHash,
+	)
+	if err == nil {
+		t.Fatal("expected missing body hash error")
+	}
+}
+
+func TestBodyHash(t *testing.T) {
+	t.Parallel()
+
+	body := []byte(`["hello world"]`)
+	bodyHash := hashcash.BodyHash(body)
+
+	if len(bodyHash) != 64 {
+		t.Fatalf(
+			"expected 64-char hex hash, got %d",
+			len(bodyHash),
+		)
+	}
+
+	// Same input should produce same hash.
+	bodyHash2 := hashcash.BodyHash(body)
+	if bodyHash != bodyHash2 {
+		t.Fatal("body hash not deterministic")
+	}
+
+	// Different input should produce different hash.
+	bodyHash3 := hashcash.BodyHash([]byte("different"))
+	if bodyHash == bodyHash3 {
+		t.Fatal("different inputs produced same hash")
+	}
+}
+
+func TestStampHash(t *testing.T) {
+	t.Parallel()
+
+	hash1 := hashcash.StampHash("stamp1")
+	hash2 := hashcash.StampHash("stamp2")
+
+	if hash1 == hash2 {
+		t.Fatal("different stamps produced same hash")
+	}
+
+	// Same input should be deterministic.
+	hash1b := hashcash.StampHash("stamp1")
+	if hash1 != hash1b {
+		t.Fatal("stamp hash not deterministic")
+	}
+}
+
+func TestMintChannelStamp(t *testing.T) {
+	t.Parallel()
+
+	bodyHash := testBodyHash()
+	stamp := hashcash.MintChannelStamp(
+		testBits, testChannel, bodyHash,
+	)
+
+	if stamp == "" {
+		t.Fatal("expected non-empty stamp")
+	}
+
+	// Validate the minted stamp.
+	validator := hashcash.NewChannelValidator()
+
+	err := validator.ValidateStamp(
+		stamp, testBits, testChannel, bodyHash,
+	)
+	if err != nil {
+		t.Fatalf("minted stamp failed validation: %v", err)
+	}
+}