Add playlist item update and reorder parity

[guidefari]

Parent PRD

#9

What to build

Fill the remaining playlist item parity gap by supporting Spotify's item update and reorder flow through /playlists/{id}/items. This slice should give consumers a complete way to mutate playlist ordering without dropping to raw request helpers.

Acceptance criteria

• Public playlist service exposes item update or reorder behavior equivalent to Spotify's /playlists/{id}/items API
• Request shaping matches Spotify's expected payload and query semantics for reorder or update operations
• Tests cover observable playlist item mutation behavior and snapshot response handling

Blocked by

None - can start immediately

User stories addressed

• User story 1
• User story 2
• User story 16
• User story 17
• User story 29
• User story 37

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Migrated from guidefari/spotify-effect#57