feat: add flowsheet entry_type discriminated union with V2 API

[jakebromberg]

Summary

Add an explicit entry_type discriminated union to flowsheet entries in the database, with V1 API compatibility and a V2 playlist endpoint for clean discriminated union responses.

Entry Types

Type	Description
track	Song play
show_start	Show begins
show_end	Show ends
dj_join	DJ joins show
dj_leave	DJ leaves show
talkset	DJ talk segment (announcements, station ID)
breakpoint	Hour marker (top of hour transitions)
message	Custom message

Database Changes

• Add flowsheet_entry_type enum
• Add entry_type column with migration and backfill
• Add index for filtering by entry type

API Changes

V1 API - Additive change only:

• All responses now include entry_type field
• No breaking changes

V2 API - Single endpoint:

GET /v2/flowsheet/playlist  - Returns show info with entries in discriminated union format

V1 vs V2 Playlist Response

V1 /flowsheet/playlist: entries include all fields

{
  "entries": [{
    "id": 123,
    "entry_type": "track",
    "artist_name": "Built to Spill",
    "message": null,
    ...
  }]
}

V2 /v2/flowsheet/playlist: entries use clean discriminated union (no irrelevant fields)

{
  "entries": [{
    "id": 123,
    "entry_type": "track",
    "artist_name": "Built to Spill",
    ...
  }]
}

Test plan

• 23 unit tests for transformToV2() function
• V2 playlist integration test
• V1 entry_type compatibility test
• Run migration on dev database
• Verify backfill: SELECT entry_type, COUNT(*) FROM flowsheet GROUP BY entry_type
• Test legacy mirror sync with MySQL database