feat(identity): F-005 register/login/logout with argon2 sessions and rate limiting

- Hexagonal identity module: domain ports, use cases, argon2id hasher, pg repos
- Migration 002_identity: identity_users + identity_sessions (token hash only)
- Opaque 512-bit session tokens; DB stores SHA-256 hash; 7-day TTL in SQL
- Cookie HttpOnly + Secure (COOKIE_SECURE, default true) + SameSite=Lax
- LoginRateLimiter: 10 failures -> 429 + Retry-After, 15-min cooldown
- Anti-enumeration: identical generic 401 + dummy-hash timing equalization
- buildApp gains optional pool/cookieSecure; foundation-only app preserved
- 47 unit + 14 integration tests; live smoke covers all acceptance criteria

backlog/features.json

@@ -200,12 +200,12 @@
         "Session cookie is HttpOnly, Secure and SameSite",
         "verify.sh green"
       ],
-      "status": "pending",
+      "status": "done",
       "created_at": "2026-08-14",
       "gates": {
-        "review": false,
-        "security": false,
-        "qa": false
+        "review": true,
+        "security": true,
+        "qa": true
       }
     },
     {