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/evidence.py

@@ -0,0 +1,51 @@
+"""Blue-team evidence files attached to a `mission_test`."""
+
+from __future__ import annotations
+
+import uuid
+from datetime import datetime
+from typing import TYPE_CHECKING
+
+from sqlalchemy import BigInteger, DateTime, ForeignKey, Index, String, Text, Uuid, func
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.db.base import Base
+from app.db.mixins import SoftDeleteMixin, TimestampMixin, UuidPkMixin
+
+if TYPE_CHECKING:
+    from app.models.auth import User
+
+
+class EvidenceFile(Base, UuidPkMixin, TimestampMixin, SoftDeleteMixin):
+    __tablename__ = "evidence_files"
+
+    mission_test_id: Mapped[uuid.UUID] = mapped_column(
+        Uuid(as_uuid=True),
+        ForeignKey("mission_tests.id", ondelete="CASCADE"),
+        nullable=False,
+    )
+    sha256: Mapped[str] = mapped_column(String(64), nullable=False)
+    mime: Mapped[str] = mapped_column(String(127), nullable=False)
+    size_bytes: Mapped[int] = mapped_column(BigInteger, nullable=False)
+    storage_path: Mapped[str] = mapped_column(Text, nullable=False)
+    original_filename: Mapped[str] = mapped_column(String(255), nullable=False)
+    uploaded_by_user_id: Mapped[uuid.UUID | None] = mapped_column(
+        Uuid(as_uuid=True),
+        ForeignKey("users.id", ondelete="SET NULL"),
+        nullable=True,
+    )
+    uploaded_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), nullable=False
+    )
+
+    uploaded_by: Mapped["User | None"] = relationship(back_populates="uploaded_evidence")
+
+    __table_args__ = (
+        Index("ix_evidence_files_mission_test", "mission_test_id"),
+        Index("ix_evidence_files_sha256", "sha256"),
+        Index(
+            "ix_evidence_files_active",
+            "deleted_at",
+            postgresql_where="deleted_at IS NULL",
+        ),
+    )