fix(api): R14 — promote route translates V1-0 ValueError to 400

POST /entities/{id}/promote now wraps the promote_entity call in
try/except ValueError → HTTPException(400). Previously the new
V1-0 provenance re-check raised ValueError that the route didn't
catch, so legacy no-provenance candidates promoted via the API
surfaced as 500 instead of 400.

Matches the existing ValueError → 400 handling on POST /entities
(api_create_entity at routes.py:1490).

Test: test_api_promote_returns_400_on_legacy_no_provenance inserts
a pre-V1-0 legacy candidate directly, POSTs to the promote route,
asserts 400 with the expected detail, asserts the row stays
candidate.

Test count: 547 -> 548. Full suite green in 72.91s.

Closes R14.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_v1_0_write_invariants.py

@@ -160,6 +160,42 @@ def test_promote_rejects_legacy_candidate_without_provenance(tmp_data_dir):
     assert got.status == "candidate"
 
 
+def test_api_promote_returns_400_on_legacy_no_provenance(tmp_data_dir):
+    """R14 (Codex, 2026-04-22): the HTTP promote route must translate
+    the V1-0 ValueError for no-provenance candidates into 400, not 500.
+    Previously the route didn't catch ValueError so legacy bad
+    candidates surfaced as a server error."""
+    init_db()
+    init_engineering_schema()
+
+    import uuid as _uuid
+    from fastapi.testclient import TestClient
+    from atocore.main import app
+
+    entity_id = str(_uuid.uuid4())
+    with get_connection() as conn:
+        conn.execute(
+            "INSERT INTO entities (id, entity_type, name, project, "
+            "description, properties, status, confidence, source_refs, "
+            "extractor_version, canonical_home, hand_authored, "
+            "created_at, updated_at) "
+            "VALUES (?, 'component', 'Legacy HTTP', 'p04-gigabit', "
+            "'', '{}', 'candidate', 1.0, '[]', '', 'entity', 0, "
+            "CURRENT_TIMESTAMP, CURRENT_TIMESTAMP)",
+            (entity_id,),
+        )
+
+    client = TestClient(app)
+    r = client.post(f"/entities/{entity_id}/promote")
+    assert r.status_code == 400
+    assert "source_refs required" in r.json().get("detail", "")
+
+    # Row still candidate — the 400 didn't half-transition.
+    got = get_entity(entity_id)
+    assert got is not None
+    assert got.status == "candidate"
+
+
 def test_promote_accepts_candidate_flagged_hand_authored(tmp_data_dir):
     """The other side of the promote re-check: hand_authored=1 with
     empty source_refs still lets promote succeed, matching