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/models/setting.py

@@ -0,0 +1,37 @@
+"""Platform settings (key/value JSONB) and admin-defined detection levels."""
+
+from __future__ import annotations
+
+from typing import Any
+
+from sqlalchemy import Integer, String, Text
+from sqlalchemy.dialects.postgresql import JSONB
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.db.base import Base
+from app.db.mixins import TimestampMixin, UuidPkMixin
+
+
+class Setting(Base, TimestampMixin):
+    __tablename__ = "settings"
+
+    key: Mapped[str] = mapped_column(String(80), primary_key=True)
+    value: Mapped[Any] = mapped_column(JSONB, nullable=False)
+    description: Mapped[str | None] = mapped_column(Text, nullable=True)
+
+
+class DetectionLevel(Base, UuidPkMixin, TimestampMixin):
+    """Custom taxonomy admin can edit (cf. spec §F6).
+
+    Pre-seeded with detected_blocked / detected_alert / logged_only / not_detected.
+    """
+
+    __tablename__ = "detection_levels"
+
+    key: Mapped[str] = mapped_column(String(40), unique=True, nullable=False)
+    label_fr: Mapped[str] = mapped_column(String(80), nullable=False)
+    label_en: Mapped[str] = mapped_column(String(80), nullable=False)
+    color_token: Mapped[str] = mapped_column(String(16), nullable=False)
+    position: Mapped[int] = mapped_column(Integer, nullable=False)
+    is_default: Mapped[bool] = mapped_column(default=False, nullable=False)
+    is_system: Mapped[bool] = mapped_column(default=False, nullable=False)