fix(review): #89 explicit metadata lock — deterministic copy on lock, not on review

The Review screen now has an explicit 'Lock & Build Title' step. Until the
owner locks, title/short_description stay empty (no 'ready' copy from an
unlocked AI draft). On lock the server validates the taxonomy, sets
metadata_locked=1, builds + persists the deterministic title/short_description
(owner prose wins), and returns the ready state.

- schema.sql: add metadata_locked INTEGER DEFAULT 0 to slabs
- db/__init__.py: bump _SCHEMA_SENTINELS for metadata_locked
- slabs.py: expose metadata_locked in GET; reject it in PUT (server-managed)
- review.py: POST /{id}/review action=lock path; regular review reports
  stored (empty) copy until locked
- test_review.py: 8 tests covering lock gate, owner-prose-wins, stale taxonomy
- frontend/api/slabs.ts: Slab.metadata_locked, ReviewResult.locked,
  ReviewInput.action, lockSlab() helper
- Review.svelte: lockMetadata() with correct commit-token ordering (bump AFTER
  inner commit so the inner PUT isn't superseded), Lock button, locked banner
- e2e/m3-create.spec.ts: pare to single golden path (capture→review→lock→
  ready→publish→done); mock respects optimistic concurrency (client_rev) so
  stale PUTs 409 like the real backend

backend/db/__init__.py

@@ -55,6 +55,7 @@ _SCHEMA_SENTINELS = [
     ("settings", "value"),
     ("slabs", "deleted_at"),
     ("slabs", "server_rev"),
+    ("slabs", "metadata_locked"),
     ("slab_photos", "original_blob"),
     ("sync_log", "payload_hash"),
 ]

backend/db/schema.sql

@@ -35,6 +35,9 @@ CREATE TABLE IF NOT EXISTS slabs (
     species_confidence REAL,
     figure_confidence REAL,
     grade_confidence REAL,
+    -- #89: explicit owner lock of the confirmed taxonomy set. Deterministic
+    -- title/short_description are built and persisted only once this is 1.
+    metadata_locked INTEGER CHECK(metadata_locked IN (0,1) OR metadata_locked IS NULL),
     inference_status TEXT CHECK(inference_status IN ('inferring','done','failed') OR inference_status IS NULL),
     calibration_photo_id TEXT,
     client_rev INTEGER NOT NULL DEFAULT 1,