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/contribution.py

@@ -50,6 +50,9 @@ class Contribution(Base, TimestampMixin):
     related_workstream_id: Mapped[uuid.UUID | None] = mapped_column(
         UUID(as_uuid=True), ForeignKey("workstreams.id", ondelete="SET NULL"), nullable=True
     )
+    repo_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("managed_repos.id", ondelete="SET NULL"), nullable=True
+    )
     submitted_at: Mapped[datetime | None] = mapped_column(
         DateTime(timezone=True), nullable=True
     )
@@ -60,3 +63,4 @@ class Contribution(Base, TimestampMixin):
 
     topic: Mapped["Topic"] = relationship("Topic", lazy="selectin")  # noqa: F821
     workstream: Mapped["Workstream"] = relationship("Workstream", lazy="selectin")  # noqa: F821
+    repo: Mapped["ManagedRepo"] = relationship("ManagedRepo", lazy="selectin")  # noqa: F821