feat(gems): three-pass schema migration aligning state-hub with GEMS

Implements CUST-WP-0007. Resolves inconsistencies I-1, I-2, I-5, I-6
identified in the GEMS audit (GenericEntityModellingSystem.md).

Pass 1 (e1f2a3b4c5d6): domain_id FK on extension_points and
technical_debt (replaces raw string column); repo_id FK on contributions.
Fixes domain-filtering bugs in EP/TD dashboard pages.

Pass 2 (f2a3b4c5d6e7): repo_id nullable FK on workstreams, aligning
the GEMS primary attachment with ADR-001 (repo > topic). Dashboard
pages updated to prefer repo->domain over topic->domain.

Pass 3 (a3b4c5d6e7f8): SBOMSnapshot container entity (GEMS Complex
between Repository and SBOMEntry). Ingest is now additive — each call
creates a new snapshot; history is retained. List/report endpoints
filter to latest snapshot per repo via _latest_snapshot_ids_subquery().
New endpoints: GET /sbom/snapshots/, GET /sbom/snapshots/{id}/.
Dashboard gains a Snapshot History section.

Also adds GEMS analysis artefacts: wiki/GEMS-StateHub-TypeRegistry.md,
wiki/GEMS-StateHub-SWOT.md, workplans/CUST-WP-0006 (analysis),
workplans/CUST-WP-0007 (migration, now completed).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/models/__init__.py

@@ -10,6 +10,7 @@ from api.models.extension_point import ExtensionPoint, EPStatus
 from api.models.technical_debt import TechnicalDebt, TDStatus
 from api.models.managed_repo import ManagedRepo
 from api.models.contribution import Contribution, ContributionType, ContributionStatus
+from api.models.sbom_snapshot import SBOMSnapshot
 from api.models.sbom_entry import SBOMEntry, Ecosystem
 
 __all__ = [
@@ -25,5 +26,6 @@ __all__ = [
     "TechnicalDebt", "TDStatus",
     "ManagedRepo",
     "Contribution", "ContributionType", "ContributionStatus",
+    "SBOMSnapshot",
     "SBOMEntry", "Ecosystem",
 ]