feat(phase-34): resolve blocking tech debt — Redis, domain exceptions, indexes, CI

Foundational changes required before any new feature work can begin.

- 0.1 Redis infrastructure: add redis:7-alpine to docker-compose dev and prod,
  REDIS_URL config, singleton client in app/infrastructure/redis_client.py
- 0.2 Token blacklist on Redis SEC-001: replace in-memory dict with Redis SETEX
  keyed by jti, auto-expiring TTL derived from token exp
- 0.3 Database indexes SR-006: Alembic migration b019 with 5 composite indexes
  for scoring, MTTD/MTTR, remediation, and notification queries
- 0.4 Domain exceptions TD-003: app/domain/exceptions.py with typed errors,
  error_handler middleware mapping them to HTTP, services decoupled from FastAPI
- 0.5 Fix silenced exceptions TD-007: replace 4 bare except-pass blocks in
  test_workflow_service with logger.warning with exc_info
- 0.6 CI pipeline TD-009: GitHub Actions workflow with Postgres and Redis
  service containers, ruff lint, pytest; ruff.toml for baseline config

backend/app/main.py

@@ -32,6 +32,8 @@ from app.routers import scores as scores_router
 from app.routers import operational_metrics as operational_metrics_router
 from app.routers import compliance as compliance_router
 from app.routers import snapshots as snapshots_router
+from app.domain.exceptions import DomainException
+from app.middleware.error_handler import domain_exception_handler
 from app.storage import ensure_bucket_exists
 from app.jobs.mitre_sync_job import start_scheduler, scheduler
 
@@ -68,6 +70,9 @@ limiter = Limiter(key_func=get_remote_address)
 app.state.limiter = limiter
 app.add_exception_handler(RateLimitExceeded, _rate_limit_exceeded_handler)
 
+# ── Domain exception → HTTP mapping ──────────────────────────────────────
+app.add_exception_handler(DomainException, domain_exception_handler)
+
 # ── CORS ──────────────────────────────────────────────────────────────────
 from app.config import settings as _settings