search: add /api/v1/search API and opencode collector parser

- Add FTS5 search repository with BM25 ranking, cursor pagination, and owner scoping
- Expose GET /api/v1/search with query, tool, host, since/until filters
- Add opencode collector parser reading from SQLite opencode.db
- Spike document recording canonical opencode storage format

Invariants: no schema migrations, no wire type changes, read-only search,
marker snippets (not HTML), invalid queries → 400 INVALID, parser interface
unchanged, collector state schema preserved, stats/react stubs untouched.

Reviewed: offset fix (message.rowid instead of time_created for progress marker),
all 22 test packages pass.