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/middleware/error_handler.py

@@ -0,0 +1,43 @@
+"""Domain exception → HTTP response mapping.
+
+This module provides a single exception handler that converts
+domain-layer exceptions into structured JSON responses, keeping
+the service layer free from FastAPI's ``HTTPException``.
+"""
+
+from fastapi import Request
+from fastapi.responses import JSONResponse
+
+from app.domain.exceptions import (
+    AuthorizationError,
+    DomainException,
+    DuplicateEntityError,
+    EntityNotFoundError,
+    InvalidOperationError,
+    InvalidTransitionError,
+)
+
+EXCEPTION_STATUS_MAP: dict[type[DomainException], int] = {
+    EntityNotFoundError: 404,
+    DuplicateEntityError: 409,
+    InvalidTransitionError: 400,
+    InvalidOperationError: 400,
+    AuthorizationError: 403,
+}
+
+
+async def domain_exception_handler(
+    request: Request,
+    exc: DomainException,
+) -> JSONResponse:
+    """Convert a :class:`DomainException` into a JSON error response."""
+    status_code = EXCEPTION_STATUS_MAP.get(type(exc), 400)
+
+    content: dict = {"detail": exc.message, "code": exc.code}
+
+    if isinstance(exc, InvalidTransitionError):
+        content["current_state"] = exc.current_state
+        content["target_state"] = exc.target_state
+        content["valid_transitions"] = exc.valid_transitions
+
+    return JSONResponse(status_code=status_code, content=content)