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>

.gitignore

@@ -14,3 +14,8 @@ venv/
 .claude/*
 !.claude/commands/
 !.claude/commands/**
+
+# Editor / IDE state — user-specific, not project config
+.obsidian/
+.vscode/
+.idea/