feat(keys): first-class BYOK key management — store, catalog, /api/keys

The auth/keys wave (2026-09-19): the server now owns the flat auth.json
map directly (LH_AUTH_FILE from main.cjs; repo-root auth.json in dev,
gitignored). The store reflects itself into the child's env at boot with
fill-gap precedence — real env (incl. .env.local in dev) always wins —
so every consumer keeps its plain process.env reads and saved keys take
effect immediately, no restart; main.cjs's spawn-time injection retired
(an env copy would shadow every settings-page change).

Curated 4-key catalog (OpenRouter, Deepgram, Google search pair) with
shape-only validation (no live pings — owner decision), 400s on bad
pastes, last-4+length masking (plaintext never leaves the store),
unknown file vars preserved + surfaced. Routes: GET /api/keys,
PUT/DELETE /api/keys/:id. Tests: catalog/store/composition, with the
env-sanitizing prologue (a dev shell may export real keys; tests use
throwaway values only).

.gitignore

@@ -10,3 +10,6 @@ SCOPE.md
 # raw real-data import sources (user directive 2026-09-13: never commit)
 real-data-extraction/
 dist/
+
+# dev auth/keys store (auth-keys wave): repo-root auth.json — secrets never committed
+auth.json