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/handlers/handlers.go

@@ -36,15 +36,21 @@ type Params struct {
 
 const defaultIdleTimeout = 30 * 24 * time.Hour
 
+// spentHashcashTTL is how long spent hashcash tokens are
+// retained for replay prevention. Per issue requirements,
+// this is 1 year.
+const spentHashcashTTL = 365 * 24 * time.Hour
+
 // Handlers manages HTTP request handling.
 type Handlers struct {
-	params        *Params
-	log           *slog.Logger
-	hc            *healthcheck.Healthcheck
-	broker        *broker.Broker
-	hashcashVal   *hashcash.Validator
-	stats         *stats.Tracker
-	cancelCleanup context.CancelFunc
+	params          *Params
+	log             *slog.Logger
+	hc              *healthcheck.Healthcheck
+	broker          *broker.Broker
+	hashcashVal     *hashcash.Validator
+	channelHashcash *hashcash.ChannelValidator
+	stats           *stats.Tracker
+	cancelCleanup   context.CancelFunc
 }
 
 // New creates a new Handlers instance.
@@ -58,12 +64,13 @@ func New(
 	}
 
 	hdlr := &Handlers{ //nolint:exhaustruct // cancelCleanup set in startCleanup
-		params:      &params,
-		log:         params.Logger.Get(),
-		hc:          params.Healthcheck,
-		broker:      broker.New(),
-		hashcashVal: hashcash.NewValidator(resource),
-		stats:       params.Stats,
+		params:          &params,
+		log:             params.Logger.Get(),
+		hc:              params.Healthcheck,
+		broker:          broker.New(),
+		hashcashVal:     hashcash.NewValidator(resource),
+		channelHashcash: hashcash.NewChannelValidator(),
+		stats:           params.Stats,
 	}
 
 	lifecycle.Append(fx.Hook{
@@ -285,4 +292,20 @@ func (hdlr *Handlers) pruneQueuesAndMessages(
 			)
 		}
 	}
+
+	// Prune spent hashcash tokens older than 1 year.
+	hashcashCutoff := time.Now().Add(-spentHashcashTTL)
+
+	pruned, err := hdlr.params.Database.
+		PruneSpentHashcash(ctx, hashcashCutoff)
+	if err != nil {
+		hdlr.log.Error(
+			"spent hashcash pruning failed", "error", err,
+		)
+	} else if pruned > 0 {
+		hdlr.log.Info(
+			"pruned spent hashcash tokens",
+			"deleted", pruned,
+		)
+	}
 }