feat(genres): allow custom track genres

[dylanjeffers]

Summary

• Discovery indexer no longer rejects tracks whose genre isn't in the hardcoded allowlist; just caps at 100 chars. The allowlist is retained as a canonical reference for read-side trending/metrics scoping.
• SDK upload schemas for tracks, albums, and playlists changed from z.enum(Genre) to z.string().min(1).max(100). SDK regenerated from updated swagger (generated Genre enum dropped). Manual Genre enum is now the canonical export from @audius/sdk with PascalCase keys, plus a new GenreString = Genre | string helper type.
• Web SelectGenreField rewritten from <SelectField> to <TextField> + native <datalist> — users can pick a known genre via autocomplete or type a custom one. maxLength=100 gives the built-in harmony character counter.

Pairs with AudiusProject/api#TBD — the Go API must ship first (or together) so it stops rejecting custom-genre POSTs before this indexer relaxation lands; otherwise tracks pass the Go validator and then hit IndexingValidationError.

What we deliberately did NOT change

• index_trending.py and get_genre_metrics.py still scope to genre_allowlist. Custom-genre tracks silently won't appear in trending or genre-metrics endpoints — intentional MVP cut; product decision for follow-up.
• packages/mobile has its own genre picker that wasn't touched — follow-up PR needed.
• No write-side normalization. "Hip-Hop" / "Hip Hop" / "hip hop" become three distinct genres. Lowercase/canonicalize is a future-PR decision.

Breaking change (browser dist only)

packages/sdk/src/sdk/types/Genre.ts keys changed from SCREAMING_SNAKE to PascalCase to match the previously-generated enum that downstream consumers (common, web, mobile) all imported. Any external integration referencing window.audiusSdk.Genre.HIP_HOP_RAP etc. via the browser dist needs to update to Genre.HipHopRap.

Test plan

• tsc --noEmit in packages/sdk — clean
• vitest run in packages/sdk — 17 files, 196 passed, 2 skipped (incl. TracksApi / AlbumsApi / PlaylistsApi suites)
• SDK regenerated via node ./src/sdk/api/generator/gen.js --spec <local-edited-swagger> against the swagger with my Go-side schema change
• Run the upload form in packages/web, type a custom genre like "Phonk", confirm it submits and the track is created
• Existing known-genre flows (pick "Electronic" from the datalist) still work
• Verify >100-char input is rejected client-side (zod) and server-side (Go validator + Python indexer)
• Confirm no regression in trending / browse-by-genre pages for canonical genres
• Mobile picker behavior unaffected on this branch (mobile uses its own field)

🤖 Generated with Claude Code

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[dylanjeffers]

Superseded by fresh-branch PR from main: #14424