fix: Session search does an extra 2000-row listing pass on every query and still silently drops valid matches

[sam-saffron-jarvis]

What changed

• Reworked session FTS search to accept structured SearchOptions instead of a bare query/limit.
• Pushed archived/category filtering into internal/session.SQLiteStore.Search(...) so SQLite filters before limiting.
• Changed the SQLite search query to return one best match per session directly, instead of returning message-level duplicates and relying on handler-side deduping.
• Expanded SearchResult with the session metadata the web handler needs, then removed the extra List(... Limit: 2000, SortByActivity: true) pass from handleSessionsSearch.
• Added regressions for:
  • older matching sessions not being dropped once more than 2000 sessions exist
  • distinct-per-session search limiting and archived/category filtering in the store

Why this is high-value

The old /v1/sessions/search path did two DB queries per request:

1. List(... Limit: 2000, SortByActivity: true) to build an allowlist map
2. Search(query, limit*4) to get raw FTS hits, then dedupe/filter in Go

That was expensive on an interactive per-keystroke path, and it still produced incorrect results:

• eligible matches older than the first 2000 listed sessions were silently excluded
• repeated hits from one transcript could consume the limit*4 window before deduping, hiding other sessions

This change removes the extra listing pass entirely and lets SQLite return already-filtered, already-deduped session matches. That cuts handler work, avoids building a 2000-entry map on every search, reduces DB work from two queries to one in the hot path, and fixes the dropped-results bug.

Validation

• gofmt -w internal/session/types.go internal/session/store.go internal/session/noop.go internal/session/sqlite.go internal/session/sqlite_test.go cmd/serve_handlers.go cmd/sessions.go cmd/serve_runtime_test.go internal/tui/sessions/browse.go internal/tui/sessions/browse_test.go cmd/serve_test.go
• go build ./...
• go test ./...
• git diff --check

New coverage added:

• TestHandleSessionsSearch_DoesNotDropOlderMatchesOutsideRecent2000ListWindow
• TestSQLiteStoreSearchReturnsDistinctFilteredSessionMatches

[SamSaffron]

Integrated the session search optimization/fix into the main working copy; closing this PR.