feat(monitor): search the permitted corpus on the server

[eric-tramel]

Summary

Monitor search currently filters the sessions already loaded in the browser. It therefore misses older retained history and can imply that a match does not exist when it is merely outside the first page.

This issue connects the Monitor to #597's server-side postings search after #599 has established paginated browsing and lazy canonical open.

Scope

• Add a Monitor API route that searches the permitted retained corpus through perf(search): use one postings path without hidden ranking retries #597.
• Preserve project/origin scope and the existing search filters.
• Return ranked hit information and the session/open locator needed to navigate to canonical content.
• Open a selected hit through the same perf(mcp): seek canonical open pages without rescanning whole sessions #598/perf(mcp,monitor): share paginated session summaries and lazy transcript loading #599 path as a browsed session.
• Keep browse pages, search results, loading, empty results, and backend errors as distinct UI states.
• Do not load the whole session corpus into the browser or create a Monitor-specific search index.

Acceptance criteria

• Search finds a permitted match whose session is absent from the initially loaded browse page.
• Ranking, snippets/hit metadata, filtering, and scope match perf(search): use one postings path without hidden ranking retries #597's repository result.
• Selecting a hit opens exact canonical content through the shared open path.
• Clearing search returns to the existing browse cursor state without claiming the loaded page is the full corpus.
• Backend failure is distinct from no matches.
• Search results contain no eager full-session transcript payload.

Chain

• Step 6 of 10.
• Previous: perf(search): use one postings path without hidden ranking retries #597.
• Next: perf(ingest): add durable budgets and resumable reconciliation for SQLite polling #601.