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/routers/state.py

@@ -255,14 +255,14 @@ async def _build_domain_summaries(session: AsyncSession) -> list[DomainSummary]:
     ):
         ws_per_domain[domain_id] = cnt
 
-    # EP counts per domain slug
+    # EP counts per domain id (via FK)
     ep_counts = {r[0]: r[1] for r in await session.execute(
-        select(ExtensionPoint.domain, func.count()).group_by(ExtensionPoint.domain)
+        select(ExtensionPoint.domain_id, func.count()).group_by(ExtensionPoint.domain_id)
     )}
 
-    # TD counts per domain slug
+    # TD counts per domain id (via FK)
     td_counts = {r[0]: r[1] for r in await session.execute(
-        select(TechnicalDebt.domain, func.count()).group_by(TechnicalDebt.domain)
+        select(TechnicalDebt.domain_id, func.count()).group_by(TechnicalDebt.domain_id)
     )}
 
     return [
@@ -271,8 +271,8 @@ async def _build_domain_summaries(session: AsyncSession) -> list[DomainSummary]:
             name=d.name,
             repo_count=repo_counts.get(d.id, 0),
             active_workstream_count=ws_per_domain.get(d.id, 0),
-            ep_count=ep_counts.get(d.slug, 0),
-            td_count=td_counts.get(d.slug, 0),
+            ep_count=ep_counts.get(d.id, 0),
+            td_count=td_counts.get(d.id, 0),
         )
         for d in domains
     ]