fix(tracks): match ISRC ignoring dashes and case, add functional index

[raymondjacobson]

Summary

ISRC lookup via GET /v1/tracks?isrc=… failed when the stored value and the query disagreed on dash placement. Partner report (Thomas):

Fixed purely on the search/query side (no normalization at write time):

• Normalize the incoming isrc query param in Go: uppercase + strip non-alphanumeric characters before passing to the DB.
• In the SQL, compare against regexp_replace(upper(isrc), '[^A-Z0-9]', '', 'g') so the stored value is normalized at compare time.

Net result: any combination of dashes/spaces/casing in either the request or the stored row matches. US-ANG-21-03742, USANG2103742, usang2103742, and US-ANG2103742 all resolve to the same row.

Index

Added tracks_isrc_normalized_idx, a partial functional index matching the comparison expression so the new query can index-scan instead of seq-scanning all of tracks:

create index concurrently if not exists tracks_isrc_normalized_idx
    on public.tracks ((regexp_replace(upper(isrc), '[^A-Z0-9]'::text, ''::text, 'g')))
    where isrc is not null;

Verified planner picks it on a 50k-row scratch table:

Bitmap Heap Scan on scratch_tracks  (cost=16.43..248.18 rows=500 width=4)
  Recheck Cond: (regexp_replace(upper(isrc), '[^A-Z0-9]'::text, ''::text, 'g'::text) = ANY (...))
  ->  Bitmap Index Scan on scratch_tracks_isrc_norm_idx  (cost=0.00..16.30 rows=500 width=0)
        Index Cond: (regexp_replace(upper(isrc), '[^A-Z0-9]'::text, ''::text, 'g'::text) = ANY (...))

Index DDL uses CREATE INDEX CONCURRENTLY (no BEGIN/COMMIT wrapper) so the build does not take an ACCESS EXCLUSIVE lock on tracks. IF NOT EXISTS keeps the migration idempotent. Partial on isrc IS NOT NULL since most rows have no ISRC.

Heads-up given #830: that PR moved 0201 away from CONCURRENTLY because the legacy Python Celery task on challenge_disbursements was keeping ~3-minute transactions open continuously, blocking the build's virtualxid wait indefinitely. tracks is written by the Go indexer and isn't subject to that long-transaction pattern, so CONCURRENTLY should complete normally here. If a stuck build needs to be aborted, drop the invalid index with DROP INDEX IF EXISTS tracks_isrc_normalized_idx; and re-run.

ISWC

There is no iswc query-param lookup today (column exists and is exposed in API responses, but no handler or sqlc query reads it). Nothing to mirror — left as-is.

Test plan

• TestGetTracksByISRC covers: stored-with-dashes queried undashed, queried with same dashes, queried lowercased; stored-without-dashes queried as-is and queried with inserted dashes.
• Existing tracks tests (TestTracksEndpoint, TestGetTracksByPermalink, TestGetTracksExcludesAccessAuthorities, Test200UnAuthed) still pass.
• Migration applies cleanly and is idempotent (re-running prints relation \"tracks_isrc_normalized_idx\" already exists, skipping).
• EXPLAIN on a seeded scratch table shows Bitmap Index Scan on tracks_isrc_normalized_idx.
• After deploy: verify SELECT indexrelid::regclass, indisvalid FROM pg_index WHERE indexrelid::regclass::text = 'tracks_isrc_normalized_idx'; returns indisvalid = true.

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