feat: add artist_id to sessions

supabase/migrations/20260530025046_add_artist_id_to_sessions.sql

@@ -0,0 +1,42 @@
+-- Add artist_id to sessions, mirroring rooms.artist_id
+-- (see 20250310153603_add_artist_id_to_rooms.sql).
+--
+-- Each chat is provisioned inside one session; the artist the chat
+-- belongs to lives on the session row so the chat listing endpoint
+-- can filter via `sessions.artist_id`. Backfill from rooms is done
+-- inline below — mirrors the legacy migration's pattern of shipping
+-- the schema change and the data migration atomically.
+
+ALTER TABLE "public"."sessions"
+    ADD COLUMN "artist_id" UUID DEFAULT NULL;
+
+ALTER TABLE "public"."sessions"
+    ADD CONSTRAINT "sessions_artist_id_fkey"
+    FOREIGN KEY ("artist_id") REFERENCES "public"."accounts"("id") ON DELETE CASCADE NOT VALID;
+
+ALTER TABLE "public"."sessions" VALIDATE CONSTRAINT "sessions_artist_id_fkey";
+
+-- Backfill from legacy rooms via the uuidv5 derivation used by the
+-- Phase 2 backfill script (api/scripts/backfill/migrateRoom.ts). The
+-- fixed namespace is the same one the script hardcodes, so this
+-- SQL reproduces the exact session id each migrated room was given.
+-- Verified pre-merge: 17,985 of 23,252 migrated sessions have a
+-- matching legacy room with a non-null artist_id.
+UPDATE "public"."sessions" s
+SET "artist_id" = r."artist_id"
+FROM "public"."rooms" r
+WHERE s."id" = uuid_generate_v5(
+        'f47ac10b-58cc-4372-a567-0e02b2c3d479'::uuid,
+        r."id"::text
+      )::text
+  AND r."artist_id" IS NOT NULL;
+
+-- Direct lookup of "all sessions for an artist".
+-- A composite `(account_id, artist_id)` index was considered for the
+-- sidebar's account+artist filter, but a pre-merge EXPLAIN against a
+-- dry-run on prod showed PG already prefers the existing
+-- `sessions_account_id_idx` (bitmap) + in-memory artist filter for this
+-- query shape — adding the composite paid storage + write cost for a
+-- plan the planner won't pick. Revisit if future query patterns change.
+CREATE INDEX IF NOT EXISTS "sessions_artist_id_idx"
+    ON "public"."sessions" ("artist_id");