[#902] Add signed webhook replay protection

[arisu6804]

Summary

Adds rotating, timestamped, nonce-bound webhook signatures while keeping the existing dispatcher API and legacy single-secret path compatible.

Security design

• Signs the exact raw body plus a fixed domain, key id, timestamp, and nonce with HMAC-SHA256.
• Uses a bounded key ring with activation and expiry windows so rotation can overlap safely.
• Enforces timestamp tolerance before verification and atomically claims a nonce only after a valid constant-time signature comparison.
• Rejects malformed, unknown-key, inactive-key, stale/future, replayed, and invalid-signature messages with stable internal reasons.
• Persists timestamp/nonce headers with deliveries so retries reuse the same signed message.
• Bounds nonce cache growth and exposes only non-secret key metadata.

Acceptance criteria

• Valid rotation keys work only within their activation/expiry windows.
• Old timestamps and reused nonces are rejected.
• Signature comparison is constant-time and failure details are not exposed by the dispatcher helper.
• Raw byte changes are rejected even when parsed JSON is equivalent.
• Tests cover rotation overlap, timestamp boundaries, replay, malformed headers, tampering, TTL, bounded eviction, and retry reuse.

Verification

• npx jest --runInBand --testMatch '**/src/services/signedWebhookSecurity.test.ts' '**/src/services/signedWebhookDispatcher.test.ts' — 17/17 passing
• ESLint on all changed TypeScript files — passing

The repository-wide build is currently blocked by a pre-existing parse error in src/routes/users.ts (line 330), and Jest reports an existing open-handle warning after the focused suites complete. No generated OpenAPI changes or unrelated fixes are included.

Closes #902