Effort 1 — Restructure src/hooks/queries → src/api (feature-sliced modules)

[chiptus]

Part of #51. Mechanical restructure, no behavior change.

What

Move src/hooks/queries/ → src/api/, flattened to one folder per feature. Per feature: a shared types.ts (entity Row type + key factory) and one self-contained use-prefixed file per endpoint holding its request/response types + fetchX/mutate + a per-file queryOptions factory + the hook consuming it. Repoint every consumer's import path. See docs/adr/0001-api-modules.md.

Per-feature checklist (leaf-first order)

• festivals
• editions (was festivals/editions)
• sets
• stages
• genres
• festival-info
• custom-links
• knowledge
• artists
• artist-notes (was artists/notes)
• groups
• invites (was groups/invites)
• voting
• auth
• admin-roles (loose useAdminRolesQuery.ts)
• invite-validation (loose useInviteValidationQuery.ts)

Done when

• All ~73 files moved; build + unit tests green; no behavior change.
• grep -rn "queryOptions(" src shows factories adopted.
• CLAUDE.md data-fetching guidance repointed to src/api/.