[tajanthind]

One focused cleanup PR (per the provider-neutral-core cleanup brief). No SDK redesign, no new providers, no capability changes, no versioning.

What changed

Provider-neutral identity (domain model + migration)

• Artist.spotify_id → Artist.external_id, Album.deezer_id/Track.deezer_id → external_id, DownloadJob.album_spotify_id → album_external_id. Values are opaque provider identities; core never parses or int()s them.
• Startup migration renames the columns on existing databases (RENAME COLUMN; data, NOT NULL/unique constraints and indexes preserved — verified on the live DB). Fresh DBs get the new names via create_all.
• SDK TrackRef gains provider-neutral external_id; deezer_id stays as a documented deprecated alias (no plugin reads it).

Deezer-specific logic removed from core workflows

• Artist search/add/sync, folder + bulk import, discography sync and the periodic scheduler now carry the opaque external id end-to-end — no int(deezer_artist_id), no isdigit() heuristics, no "resolve Deezer ID".
• Indexing/pruning/ISRC lookups/dedup use the external id generically. MetadataService docstrings no longer name any provider authoritative.
• Deezer-specific conversion/validation stays in fnack.deezer-batch (it int()s its own ids and declines foreign ids gracefully — the chain moves on).
• Legacy wire compat isolated + documented: /set-external-id also answers the old /set-deezer-id path/body key; import accepts deezer_id as an alias of external_id; candidate suggestions now emit suggested_external_id (suggested_deezer kept as alias) and the frontend was updated to the neutral keys.
• Manual-match user-pasted Deezer URLs + provider-URL help text = user-facing input handling; lookups go through the capability chain (annotated).

Tests

• New tests/architecture/test_provider_neutral_identity.py (Tests A–D): fake non-integer (fake-artist-123) and colon-prefixed (artist:abc:def) identities flow through search/discography; the Deezer plugin converts ids inside the plugin and declines foreign ones; provider removal leaves the chain working; zero providers → CapabilityUnavailable, no hidden Deezer fallback.
• Background app-context regression (real _process_track_job on a ThreadPoolExecutor worker with a settings-reading provider) unchanged + green; the structural test still covers all three protected entry points.
• Architecture suite 22/22, smoke PASSED, fnack-plugins parity OK (bundled plugins untouched, byte-identical).

Live verification

Container rebuilt from this code: the live DB migrated in place; Arctic Monkeys was added via search → discography synced (32 albums / 150 tracks) through the opaque-external-id path → "Do I Wanna Know?" downloaded, verified, and accepted through the queue worker (ISRC lookup + settings read from the worker thread).

Remaining provider mentions in core (all classified)

• Factual plugin-arrangement comments ("core has no Deezer implementation").
• Documented legacy wire aliases + the old route path (migration compat).
• User-facing manual-match provider-URL text + parsing.
• SDK deezer_id deprecated alias + TrackRef comment examples.
• The essential-plugin packaging doc (plugin catalog descriptions).
• Migration table column names.