Full-text chat search: a ⌘K palette over every visible conversation (#509)

Adds search across the content of every conversation the viewer can see — not just titles — from a ⌘K/Ctrl+K palette; Enter opens the matched conversation, and ⌘/Ctrl+Enter starts a new chat with the query as the prompt, so a fruitless search becomes a question. The session store gains searchEntries backed by Postgres full-text search: an immutable text-extraction helper with a partial GIN expression index over user/assistant/text entries, and prefix-match tsquery so results feel type-ahead; the memory store mirrors the semantics for tests and dev. The SQL joins participants and enforces the same visibility-window predicate the transcript uses, with viewer-level filtering applied again at the app layer, and the route is exposed user-scoped as GET /v1/sessions/search. The palette is debounced, aborts stale fetches, groups hits by session, highlights terms, and is keyboard-first.

**Deployment notes**

MIGRATION
Release-note the one-time index build on session_entries; large deployments may want to
pre-create the index CONCURRENTLY before upgrading
Confirm the safe_json boot DDL is already present upstream (it predates this change in the
fork)
New GIN full-text index on session_entries created at boot via plain CREATE INDEX IF NOT
EXISTS (not CONCURRENTLY) in the pg store's boot DDL — idempotent, but on a deployment
with a large session_entries table the build blocks writes to that table and delays the new
instance's boot for the build duration; blue-green reads keep working on the old instance, writes
stall
New plpgsql function entry_search_text created with CREATE OR REPLACE (idempotent); the
search SQL also calls the pre-existing safe_json() boot function — verify upstream already carries
that DDL before this lands
New endpoint GET /v1/sessions/search + web-ui /api/search: during the mixed window a new
web-ui against an old core gets 404 (palette shows no results; no data risk)
Rollback safe: old code ignores the index and function; no data rewrite, old entries are searchable
as-is

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