[rooms] improve performance of rooms query

apps/rooms/migrations/0014_room_listable.sql

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+-- The room flags every public feed filters on, as indexed columns.
+--
+-- Generated from packages/domain/src/rooms-db.ts (ROOM_SCHEMA_DDL) — keep in sync.
+--
+-- `is_dorm` was already extracted back in 0001, but `Accessibility` and `ExcludeFromLists`
+-- were not, so "the rooms a feed may show" could only be asked in JS. Every feed — hot, recommended,
+-- featured, new, recently-updated, similar, search, autocomplete — therefore SELECTed the
+-- whole `room` table and threw most of it away after parsing it. On this database that is
+-- 1173 rows and ~1.7MB of blob read to rank the 109 rooms that are actually listable: the
+-- rest are DORMS (one per account, private by construction) and other private rooms. It is
+-- the read D1 reports as slow, and it gets worse with every account that signs up.
+--
+-- VIRTUAL like the other generated columns, so the blob stays the only copy of the value
+-- and no backfill is needed — they are computed on read from the JSON already stored.
+ALTER TABLE room ADD COLUMN accessibility INTEGER GENERATED ALWAYS AS (json_extract(data, '$.Accessibility')) VIRTUAL;
+ALTER TABLE room ADD COLUMN exclude_from_lists INTEGER GENERATED ALWAYS AS (json_extract(data, '$.ExcludeFromLists')) VIRTUAL;
+
+-- A PARTIAL index: it holds only the public, non-dorm rooms, which is the small minority
+-- the feeds serve from. Scanning it visits those rows alone, so the feeds stop reading the
+-- dorms at all rather than reading and discarding them. EXPLAIN QUERY PLAN over the feed
+-- query goes from
+--
+--   SCAN room
+--
+-- to
+--
+--   SCAN room USING INDEX idx_room_public
+--
+-- `ExcludeFromLists` is deliberately NOT in the WHERE: search reads the public rooms
+-- WITHOUT that term (a room can opt out of the browse feeds and still be findable by
+-- name), so indexing the wider set lets both reads use this one index — the extra term is
+-- then checked against the handful of rows it already fetched.
+CREATE INDEX IF NOT EXISTS idx_room_public ON room (room_id)
+  WHERE is_dorm IS NOT 1 AND accessibility = 1;