#628 feat(sessions): accept artistId on POST /api/sessions

Threads optional artistId through POST /api/sessions so new sessions can be linked to the artist they were created in. Backs the sidebar's artist filter once the chat client passes it through.

• validateCreateSessionBody accepts artistId (UUID, optional).
• buildSessionInsertRow writes artist_id from artistId (defaults to null).
• createSessionHandler forwards the validated artistId into the insert.
• toSessionResponse exposes artistId on the camelCase response.
• types/database.types.ts updated with sessions.artist_id (Row/Insert/Update + sessions_artist_id_fkey relationship) — mirrors what pnpm update-types would emit after database#27 applies.

Depends on database#27 applying to prod for the new column to actually persist. Until then, inserts will fail at the DB layer; the types/code shape is correct and waits.

Test plan

• After database#27 applies, POST /api/sessions { artistId: "<uuid>" } returns { session: { artistId: "<uuid>", ... }, chat }.
• POST /api/sessions {} returns { session: { artistId: null, ... }, chat }.
• POST /api/sessions { artistId: "not-a-uuid" } returns 400.

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Summary by cubic

Adds optional artistId to POST /api/sessions and exposes it on session reads. Threads artistId through validation, insert, and responses; updates GET session test to expect artistId.

• New Features

  • Accepts artistId (UUID, optional) in request body.
  • Persists artist_id on sessions (defaults to null).
  • Returns artistId on session responses (create and read).

• Migration

  • Depends on database#27 adding sessions.artist_id and its FK. Apply before deploy.
  • Types updated to match the post-migration schema; regenerating should be a no-op afterward.

Summary by CodeRabbit

• New Features
  • Sessions now support optional artist association. When creating a session, you can specify an artist ID, which will be stored and included in session responses. The artist ID field is optional and defaults to null if not provided.