feat(m1): DB schema, migrations, diag visibility

23 tables + alembic_version covering the v1 data model:
- Auth/RBAC (8): users, groups, permissions, user_groups, group_permissions,
  invitations, invitation_groups, refresh_tokens.
- MITRE (4): mitre_tactics, mitre_techniques, mitre_subtechniques + the
  technique↔tactic many-to-many.
- Templates (4): test_templates, test_template_mitre_tags (3 nullable FKs +
  CHECK exactly_one_mitre_fk), scenario_templates, scenario_template_tests
  (UUID PK + UNIQUE(scenario_id, position) so a test can appear at multiple
  positions).
- Missions (6): missions, mission_members, mission_scenarios, mission_tests,
  mission_test_mitre_tags (deliberately denormalised — copies external_id +
  name + url, no FK to mitre_* — so a re-sync of the catalogue can't purge
  historical tags), mission_categories.
- Evidence/settings/notifications (5): evidence_files, settings (JSONB
  value), detection_levels, notifications.

SQLAlchemy 2.x with Mapped[]/mapped_column(), pk_/fk_/ck_/uq_/ix_ naming
convention. Reusable mixins (UuidPkMixin, TimestampMixin, SoftDeleteMixin —
no auto __table_args__ since classes silently clobber the mixin's).

Soft delete: deleted_at + partial indexes ix_<table>_active WHERE deleted_at
IS NULL on 9 tables (users, groups, test_templates, scenario_templates,
missions, mission_scenarios, mission_tests, mission_categories,
evidence_files). Notifications gets ix_..._unread WHERE read_at IS NULL.

CHECK constraints for status / state / opsec_level / mitre_kind enums.

New API endpoint GET /api/v1/diag/db: returns alembic_revision (short hash)
and the public-schema table_count. 503 with {"reachable": false} on a DB
outage. Database card on the SPA home consumes it.

Test stage in backend/Dockerfile (--target test): runtime + dev extras +
tests/. New make test-api spins an ephemeral pytest container against the
live DB on the compose network. backend/tests/test_schema.py: 8 integration
tests (tables, FK pairs, CHECK constraints, partial indexes, alembic-at-head,
negative INSERT proving the exactly_one_mitre_fk CHECK fires).

e2e/tests/m1-db.spec.ts: 4 Playwright tests covering the diag endpoint
contract + the Database card + footer/roadmap labels.

DoD: make clean && make up && make migrate → 23 tables, 32 FKs, 9 CHECKs,
make test-api → 9 passed, make e2e → 12 passed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/db/session.py

@@ -0,0 +1,47 @@
+"""Engine + sessionmaker. Lazily initialised so test code can swap the URL."""
+
+from __future__ import annotations
+
+from collections.abc import Iterator
+from contextlib import contextmanager
+
+from sqlalchemy import create_engine
+from sqlalchemy.engine import Engine
+from sqlalchemy.orm import Session, sessionmaker
+
+from app.core.config import settings
+
+_engine: Engine | None = None
+_SessionLocal: sessionmaker[Session] | None = None
+
+
+def get_engine() -> Engine:
+    global _engine
+    if _engine is None:
+        _engine = create_engine(
+            settings.database_url,
+            pool_pre_ping=True,
+            future=True,
+        )
+    return _engine
+
+
+def get_sessionmaker() -> sessionmaker[Session]:
+    global _SessionLocal
+    if _SessionLocal is None:
+        _SessionLocal = sessionmaker(bind=get_engine(), expire_on_commit=False, future=True)
+    return _SessionLocal
+
+
+@contextmanager
+def session_scope() -> Iterator[Session]:
+    """Context manager that commits on success, rolls back on error."""
+    s = get_sessionmaker()()
+    try:
+        yield s
+        s.commit()
+    except Exception:
+        s.rollback()
+        raise
+    finally:
+        s.close()