feat(backend): add §8 data model + Alembic baseline (B0.2, B0.3)

- SQLAlchemy 2 typed mapped classes for every spec §8 aggregate:
  engagement, c2_credential, host, user, group, group_permission,
  user_group, engagement_member, ttp, ttp_version, scenario,
  scenario_step, run, run_step, run_step_cleanup, detection, evidence,
  report, soc_session, audit_log.
- Shared mixins: UuidPkMixin (PG_UUID(as_uuid=True)) + TimestampsMixin.
- StrEnum types covering every spec enum (C2Type, PayloadType, UserType,
  EngagementStatus, HostStatus, TtpSource, RunStatus, RunStepStatus,
  CleanupStatus, DetectionLevel, DetectionSource, EvidenceStatus).
- Alembic baseline migration 202605210001_initial_schema: creates every
  table, enum, index, and idempotent grants for the audit_log
  write-only Postgres role (mimic_audit_writer).
- Audit log carries prev_hash / row_hash from v1 (D-009).
- ttp_version table coexists with run.snapshot_json (D-008,
  overrides H32).

backend/src/mimic/db/__init__.py

@@ -0,0 +1,5 @@
+"""Database layer: SQLAlchemy 2 declarative base, models, repositories."""
+
+from mimic.db.base import Base
+
+__all__ = ["Base"]