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

Squash-merge of branch claude/r14-promote-400 (3888db9), approved by
Codex (no findings, targeted suite 15 passed).

POST /entities/{id}/promote now wraps promote_entity in
try/except ValueError → HTTPException(400). Previously the 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 (routes.py:1490).

New regression test test_api_promote_returns_400_on_legacy_no_provenance
inserts a pre-V1-0 candidate directly, POSTs promote, asserts 400
with the expected detail, asserts the row stays candidate.

Also adds .obsidian/, .vscode/, .idea/ to .gitignore so editor
state doesn't sneak into future commits.

Test count: 547 → 548. 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/