feat: implement hashcash proof-of-work for session creation (#63)

## Summary

Implement SHA-256-based hashcash proof-of-work for `POST /session` to prevent abuse via rapid session creation.

closes #11

## What Changed

### Server
- **New `internal/hashcash` package**: Validates hashcash stamps (format, difficulty bits, date/expiry, resource, replay prevention via in-memory spent set with TTL pruning)
- **Config**: `NEOIRC_HASHCASH_BITS` env var (default 20, set to 0 to disable)
- **`GET /api/v1/server`**: Now includes `hashcash_bits` field when > 0
- **`POST /api/v1/session`**: Validates `X-Hashcash` header when hashcash is enabled; returns HTTP 402 for missing/invalid stamps

### Clients
- **Web SPA**: Fetches `hashcash_bits` from `/server`, computes stamp using Web Crypto API (`crypto.subtle.digest`) with batched parallelism (1024 hashes/batch), shows "Computing proof-of-work..." feedback
- **CLI (`neoirc-cli`)**: `CreateSession()` auto-fetches server info and computes a valid hashcash stamp when required; new `MintHashcash()` function in the API package

### Documentation
- README updated with full hashcash documentation: stamp format, computing stamps, configuration, difficulty table
- Server info and session creation API docs updated with hashcash fields/headers
- Roadmap updated (hashcash marked as implemented)

## Stamp Format

Standard hashcash: `1:bits:YYMMDD:resource::counter`

The SHA-256 hash of the entire stamp string must have at least `bits` leading zero bits.

## Validation Rules
- Version must be `1`
- Claimed bits ≥ required bits
- Resource must match server name
- Date within 48 hours (not expired, not too far in future)
- SHA-256 hash has required leading zero bits
- Stamp not previously used (replay prevention)

## Testing
- All existing tests pass (hashcash disabled in test config with `HashcashBits: 0`)
- `docker build .` passes (lint + test + build)

<!-- session: agent:sdlc-manager:subagent:f98d712e-8a40-4013-b3d7-588cbff670f4 -->

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Co-authored-by: clawbot <clawbot@noreply.eeqj.de>
Co-authored-by: user <user@Mac.lan guest wan>
Co-authored-by: Jeffrey Paul <sneak@noreply.example.org>
Reviewed-on: #63
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

web/src/app.jsx

@@ -8,6 +8,56 @@ const MEMBER_REFRESH_INTERVAL = 10000;
 const ACTION_PREFIX = "\x01ACTION ";
 const ACTION_SUFFIX = "\x01";
 
+// Hashcash proof-of-work helpers using Web Crypto API.
+
+function checkLeadingZeros(hashBytes, bits) {
+  let count = 0;
+  for (let i = 0; i < hashBytes.length; i++) {
+    if (hashBytes[i] === 0) {
+      count += 8;
+      continue;
+    }
+    let b = hashBytes[i];
+    while ((b & 0x80) === 0) {
+      count++;
+      b <<= 1;
+    }
+    break;
+  }
+  return count >= bits;
+}
+
+async function mintHashcash(bits, resource) {
+  const encoder = new TextEncoder();
+  const now = new Date();
+  const date =
+    String(now.getUTCFullYear()).slice(2) +
+    String(now.getUTCMonth() + 1).padStart(2, "0") +
+    String(now.getUTCDate()).padStart(2, "0");
+  const prefix = `1:${bits}:${date}:${resource}::`;
+  let nonce = Math.floor(Math.random() * 0x100000);
+  const batchSize = 1024;
+
+  for (;;) {
+    const stamps = [];
+    const hashPromises = [];
+    for (let i = 0; i < batchSize; i++) {
+      const stamp = prefix + (nonce + i).toString(16);
+      stamps.push(stamp);
+      hashPromises.push(
+        crypto.subtle.digest("SHA-256", encoder.encode(stamp)),
+      );
+    }
+    const hashes = await Promise.all(hashPromises);
+    for (let i = 0; i < hashes.length; i++) {
+      if (checkLeadingZeros(new Uint8Array(hashes[i]), bits)) {
+        return stamps[i];
+      }
+    }
+    nonce += batchSize;
+  }
+}
+
 function api(path, opts = {}) {
   const token = localStorage.getItem("neoirc_token");
   const headers = {
@@ -60,12 +110,16 @@ function LoginScreen({ onLogin }) {
   const [motd, setMotd] = useState("");
   const [serverName, setServerName] = useState("NeoIRC");
   const inputRef = useRef();
+  const hashcashBitsRef = useRef(0);
+  const hashcashResourceRef = useRef("neoirc");
 
   useEffect(() => {
     api("/server")
       .then((s) => {
         if (s.name) setServerName(s.name);
         if (s.motd) setMotd(s.motd);
+        hashcashBitsRef.current = s.hashcash_bits || 0;
+        if (s.name) hashcashResourceRef.current = s.name;
       })
       .catch(() => {});
     const saved = localStorage.getItem("neoirc_token");
@@ -81,9 +135,22 @@ function LoginScreen({ onLogin }) {
     e.preventDefault();
     setError("");
     try {
+      let hashcashStamp = "";
+      if (hashcashBitsRef.current > 0) {
+        setError("Computing proof-of-work...");
+        hashcashStamp = await mintHashcash(
+          hashcashBitsRef.current,
+          hashcashResourceRef.current,
+        );
+        setError("");
+      }
+      const reqBody = { nick: nick.trim() };
+      if (hashcashStamp) {
+        reqBody.pow_token = hashcashStamp;
+      }
       const res = await api("/session", {
         method: "POST",
-        body: JSON.stringify({ nick: nick.trim() }),
+        body: JSON.stringify(reqBody),
       });
       localStorage.setItem("neoirc_token", res.token);
       onLogin(res.nick);