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/logger.ts

@@ -0,0 +1,33 @@
+/**
+ * Structured JSON logger.
+ *
+ * One line per log event. Production observability tools (Datadog,
+ * Loki, etc.) can ingest these directly. Dev readability is
+ * secondary — if you're eyeballing, pipe through `jq`.
+ */
+
+type LogLevel = "debug" | "info" | "warn" | "error";
+
+interface LogContext {
+  [key: string]: unknown;
+}
+
+function emit(level: LogLevel, msg: string, ctx: LogContext = {}): void {
+  const entry = {
+    ts: new Date().toISOString(),
+    level,
+    component: "broker",
+    msg,
+    ...ctx,
+  };
+  // Single line, no pretty-printing. stderr so stdout is free for
+  // any app-level protocol chatter.
+  console.error(JSON.stringify(entry));
+}
+
+export const log = {
+  debug: (msg: string, ctx?: LogContext) => emit("debug", msg, ctx),
+  info: (msg: string, ctx?: LogContext) => emit("info", msg, ctx),
+  warn: (msg: string, ctx?: LogContext) => emit("warn", msg, ctx),
+  error: (msg: string, ctx?: LogContext) => emit("error", msg, ctx),
+};