feat(F-002): database foundation with migrations and dev compose

- node-pg-migrate + pg: baseline migration (extensions, app_meta) with working down
- src/infrastructure/db fail-fast pool and typed query helper
- docker-compose: postgres:16-alpine + redis:7-alpine with one-command up
- table naming convention <module>_<table> documented in README
- integration tests (6) against real PostgreSQL; strict identifier validation
  for test DDL after security-gate hardening round
- deps justified in spec/tech.md; all gates approved; verify.sh green

backlog/features.json

@@ -87,12 +87,12 @@
         "Dev PostgreSQL and Redis start with one command",
         "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
       }
     },
     {