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).
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