Search Chats Tiered

Ranked, tiered search over the caller's conversations (TOS-3459).

GET /api/chats?q= remains the BROWSE surface: a boolean substring filter over a recency-ordered keyset page, still used for infinite scroll and the "Show all" fallback. This endpoint is the SEARCH surface, and differs in three ways that the old one could not express:

• Ranked by match quality, not recency. An exact title match outranks a newer partial one; recency is only the tiebreak.
• People and agents are their own tier, grouped one row per identity with a conversation count, rather than scattered across title-less rows.
• Near-misses are reachable. council finds AI4O General Counsel via trigram similarity, in a tier explicitly labelled as a suggestion and returned only when the exact tiers are thin.

Archived conversations are included and flagged archived rather than excluded - a search is an explicit retrieval intent, so "it exists but is filed away" beats "no matches". This does not change the archived/unarchived tabs, which still scope browsing.

Makes no AI call: pg_trgm similarity and SQL ordering only.