perf(doi): fingerprint-based DB cache for DoI results

Adds doi_cache table (migration k8f9a0b1c2d3). Results are stored after
each evaluation and reused on subsequent requests when the fingerprint
matches. Fingerprint covers repo.updated_at, latest TPSC snapshot_at,
latest goal updated_at, and mtime of SCOPE.md / CLAUDE.md / tpsc.yaml.

Behaviour:
- Summary (warm cache, nothing changed): ~0.4s (was 0.9s)
- Summary (one repo stale): ~0.9s (only stale repos recomputed)
- Single repo (cache hit): ~0.2s (was 40s for full check)
- Single repo ?force_refresh=true: ~2s (full C7/C13 subprocess check)

Total journey: 108s (original) → 6s → <1s → 0.2s (cached single repo)

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

api/models/__init__.py

@@ -18,6 +18,7 @@ from api.models.agent_message import AgentMessage
 from api.models.capability_catalog import CapabilityCatalog
 from api.models.capability_request import CapabilityRequest
 from api.models.tpsc import TPSCCatalog, TPSCSnapshot, TPSCEntry
+from api.models.doi_cache import DOICache
 
 __all__ = [
     "Base",
@@ -40,4 +41,5 @@ __all__ = [
     "CapabilityCatalog",
     "CapabilityRequest",
     "TPSCCatalog", "TPSCSnapshot", "TPSCEntry",
+    "DOICache",
 ]