Fix hash ID decoding in playlist contents and grant request bodies

[rickyrombo]

Summary

• Adds trashid.IntId type: accepts hash IDs or raw ints on JSON input, always marshals as a plain integer. Designed for chain metadata fields where the indexer expects numeric IDs.
• PlaylistTrackInfo.TrackId (string → trashid.IntId): playlist_contents track IDs are now decoded at the API boundary. Previously hash ID strings were forwarded raw to chain, requiring an indexer workaround.
• addManagerBody.ManagerUserId (string → trashid.HashId): removes redundant manual DecodeHashId call, adds struct tag validation
• approveGrantBody.GrantorUserId (string → trashid.HashId): removes redundant manual DecodeHashId call, adds struct tag validation

Test plan

• POST /v1/playlists with hash ID track IDs in playlist_contents — verify chain metadata contains int IDs
• POST /v1/playlists with raw int track IDs in playlist_contents — verify still works (backward compat)
• POST /v1/users/:id/managers with hash ID manager_user_id — verify grant created
• POST /v1/users/:id/managers with missing/zero manager_user_id — verify 400
• POST /v1/users/:id/approve-grant with hash ID grantor_user_id — verify grant approved
• POST /v1/users/:id/approve-grant with missing/zero grantor_user_id — verify 400
• Unit tests for trashid.IntId marshal/unmarshal round-trip

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates API request decoding so hash IDs are converted at the API boundary for playlist contents and grant-related request bodies, ensuring downstream chain/indexer metadata uses the expected numeric IDs.

Changes:

• Introduces trashid.IntId to accept either hash IDs or raw integers on JSON input, while always marshaling as a JSON number.
• Updates playlist playlist_contents[].track_id to use trashid.IntId so track IDs are emitted as integers in chain metadata.
• Switches manager_user_id / grantor_user_id request fields to trashid.HashId, removing redundant manual decode calls.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
trashid/hashid.go	Adds IntId JSON (un)marshal behavior to normalize IDs to integers for chain metadata.
api/v1_playlist.go	Changes playlist contents track IDs to trashid.IntId so metadata contains numeric track_ids.
api/v1_grants.go	Changes grant request bodies to typed hash IDs and removes manual decoding logic.

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