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/alembic/env.py

@@ -0,0 +1,76 @@
+"""Alembic environment.
+
+We bypass `alembic.ini`'s `sqlalchemy.url` and pull the URL from the project's
+Pydantic settings so a single .env governs both runtime and migrations.
+
+Importing `app.models` registers every model on `Base.metadata`.
+"""
+
+from __future__ import annotations
+
+from logging.config import fileConfig
+
+from alembic import context
+from sqlalchemy import engine_from_config, pool
+
+# noqa: F401 — registers models on Base.metadata
+from app import models as _models  # noqa: F401
+from app.core.config import settings
+from app.db.base import Base
+
+config = context.config
+
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
+
+# Inject the DB URL at runtime.
+config.set_main_option("sqlalchemy.url", settings.database_url)
+
+target_metadata = Base.metadata
+
+
+def include_object(_object, _name, type_, _reflected, _compare_to):  # type: ignore[no-untyped-def]
+    """Skip alembic's internal version table from autogenerate diffs."""
+    if type_ == "table" and _name == "alembic_version":
+        return False
+    return True
+
+
+def run_migrations_offline() -> None:
+    """Generate SQL without an engine — useful for review."""
+    context.configure(
+        url=config.get_main_option("sqlalchemy.url"),
+        target_metadata=target_metadata,
+        literal_binds=True,
+        dialect_opts={"paramstyle": "named"},
+        include_object=include_object,
+        compare_type=True,
+        compare_server_default=True,
+    )
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def run_migrations_online() -> None:
+    """Apply migrations against a live DB."""
+    connectable = engine_from_config(
+        config.get_section(config.config_ini_section, {}),
+        prefix="sqlalchemy.",
+        poolclass=pool.NullPool,
+    )
+    with connectable.connect() as connection:
+        context.configure(
+            connection=connection,
+            target_metadata=target_metadata,
+            include_object=include_object,
+            compare_type=True,
+            compare_server_default=True,
+        )
+        with context.begin_transaction():
+            context.run_migrations()
+
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    run_migrations_online()