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/doi_cache.py

@@ -0,0 +1,27 @@
+import uuid
+from datetime import datetime
+from sqlalchemy import Boolean, DateTime, ForeignKey, String, Text, func
+from sqlalchemy.dialects.postgresql import JSON, UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from api.models.base import Base
+
+
+class DOICache(Base):
+    __tablename__ = "doi_cache"
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    repo_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("managed_repos.id", ondelete="CASCADE"),
+        nullable=False, unique=True, index=True,
+    )
+    tier: Mapped[str] = mapped_column(String(20), nullable=False)
+    core_pass: Mapped[bool] = mapped_column(Boolean, nullable=False, server_default="false")
+    standard_pass: Mapped[bool] = mapped_column(Boolean, nullable=False, server_default="false")
+    full_pass: Mapped[bool] = mapped_column(Boolean, nullable=False, server_default="false")
+    criteria: Mapped[list | None] = mapped_column(JSON, nullable=True)
+    # Pipe-joined string of timestamps/mtimes used to detect staleness
+    fingerprint: Mapped[str] = mapped_column(Text, nullable=False)
+    checked_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())
+    updated_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now())