feat(broker): production hardening — caps, limits, metrics, logging

Adds the minimum ops surface area for a production broker without
over-engineering. All new config knobs are env-var driven with sane
defaults.

New modules:
- logger.ts: structured JSON logs (one line, stderr, ready for
  Loki/Datadog ingestion without preprocessing)
- metrics.ts: in-process Prometheus counters + gauges, exposed at
  GET /metrics. Tracks connections, messages, queue depth, TTL
  sweeps, hook requests, DB health.
- rate-limit.ts: token-bucket rate limiter keyed by (pid, cwd).
  Applied to POST /hook/set-status at 30/min default.
- db-health.ts: Postgres ping loop with exponential-backoff retry.
  GET /health returns 503 while DB is down.
- build-info.ts: version + gitSha (from GIT_SHA env or `git rev-parse`
  fallback) + uptime, surfaced on /health.

Behavior changes:
- Connection caps: MAX_CONNECTIONS_PER_MESH (default 100). Exceed →
  close(1008, "capacity") + metric increment.
- Message size: MAX_MESSAGE_BYTES (default 65536). WS applies it via
  `ws.maxPayload`. Hook POST bodies cap out with 413.
- Structured logs everywhere replacing the old `log()` helper.
- Env validation stricter: DATABASE_URL required + regex-checked for
  postgres:// prefix.

New endpoints:
- GET /health → {status, db, version, gitSha, uptime}. 503 if DB down.
- GET /metrics → Prometheus text format.

Verified: 21/21 tests still pass. Hit /health + /metrics live —
gitSha resolves correctly via `git rev-parse --short HEAD` in dev.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/broker/src/rate-limit.ts

@@ -0,0 +1,61 @@
+/**
+ * Token-bucket rate limiter keyed by an arbitrary string.
+ *
+ * Used to cap POST /hook/set-status at a sane per-session rate
+ * (hook scripts legitimately fire every turn; anything faster is
+ * either a loop or a compromised agent).
+ *
+ * In-process only. If we scale to multiple broker instances this
+ * moves to Redis, but for the single-instance broker it's enough.
+ */
+
+interface Bucket {
+  tokens: number;
+  lastRefill: number;
+}
+
+export class TokenBucket {
+  private buckets = new Map<string, Bucket>();
+  private readonly refillPerMs: number;
+
+  constructor(
+    private capacity: number,
+    refillPerMinute: number,
+  ) {
+    this.refillPerMs = refillPerMinute / 60_000;
+  }
+
+  /** Take one token. Returns true if allowed, false if rate-limited. */
+  take(key: string, now = Date.now()): boolean {
+    const bucket = this.buckets.get(key) ?? {
+      tokens: this.capacity,
+      lastRefill: now,
+    };
+    const elapsed = now - bucket.lastRefill;
+    if (elapsed > 0) {
+      bucket.tokens = Math.min(
+        this.capacity,
+        bucket.tokens + elapsed * this.refillPerMs,
+      );
+      bucket.lastRefill = now;
+    }
+    if (bucket.tokens < 1) {
+      this.buckets.set(key, bucket);
+      return false;
+    }
+    bucket.tokens -= 1;
+    this.buckets.set(key, bucket);
+    return true;
+  }
+
+  /** Periodic GC: drop buckets whose keys haven't been touched in a while. */
+  sweep(olderThanMs = 10 * 60 * 1000, now = Date.now()): void {
+    for (const [key, bucket] of this.buckets) {
+      if (now - bucket.lastRefill > olderThanMs) this.buckets.delete(key);
+    }
+  }
+
+  get size(): number {
+    return this.buckets.size;
+  }
+}