Add alternative names and cover art for soundtracks

Alternative names (#32):
- PUT /api/v1/soundtracks/{uuid}/alternative-names replaces a soundtrack's
  alternative names (trimmed, deduplicated, without the soundtrack's own name).
- alternative_names is in the soundtrack and statistics responses.
- /find matches them too and shows which name matched.

Covers (#12), stored in COVERS_PATH and off when it is not set:
- GET, PUT (upload) and DELETE /api/v1/soundtracks/{uuid}/cover, and
  PUT .../cover/source to pick a SteamGridDB game or AniList anime by ID.
  Uploaded and picked covers are never replaced automatically.
- With STEAMGRIDDB_API_KEY, a background job searches SteamGridDB, then
  AniList, by name, alternative names and the name without bracketed parts,
  at startup, after a sync and after names change. Search results are ranked
  by matching words, so "Civilization IV" gets "Sid Meier's Civilization IV".
- cover_modified (Unix seconds) is in the soundtrack and statistics responses,
  so clients can cache covers.

The new columns are added to migration 000006, which is not released yet.

Closes #12, closes #32

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/repository/querier.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"github.com/google/uuid"
+	"github.com/jackc/pgx/v5/pgtype"
 )
 
 type Querier interface {
@@ -23,6 +24,8 @@ type Querier interface {
 	AddSoundtrackPlayedByUUID(ctx context.Context, argUuid uuid.UUID) error
 	CheckSong(ctx context.Context, arg CheckSongParams) (int64, error)
 	CheckSongWithHash(ctx context.Context, hash string) (int64, error)
+	// Removes the cover, so the job searches for one again.
+	ClearCover(ctx context.Context, argUuid uuid.UUID) error
 	ClearSongs(ctx context.Context) error
 	ClearSongsBySoundtrackId(ctx context.Context, soundtrackID int32) error
 	ClearSoundtracks(ctx context.Context) error
@@ -73,14 +76,21 @@ type Querier interface {
 	InsertSongInList(ctx context.Context, arg InsertSongInListParams) error
 	InsertSoundtrack(ctx context.Context, arg InsertSoundtrackParams) (InsertSoundtrackRow, error)
 	InsertSoundtrackWithExistingId(ctx context.Context, arg InsertSoundtrackWithExistingIdParams) error
+	ListCoverFiles(ctx context.Context) ([]string, error)
 	// The song columns are NULL when the song has been removed from the library since.
 	ListQueueSongs(ctx context.Context, queueUuid uuid.UUID) ([]ListQueueSongsRow, error)
 	ListSessions(ctx context.Context) ([]Session, error)
+	// Active soundtracks without a cover that the job has not searched since checked_before.
+	ListSoundtracksNeedingCover(ctx context.Context, checkedBefore pgtype.Timestamptz) ([]ListSoundtracksNeedingCoverRow, error)
+	MarkCoverChecked(ctx context.Context, argUuid uuid.UUID) error
 	MarkSoundtracksDeletedExcept(ctx context.Context, ids []int32) error
 	RemoveBrokenSong(ctx context.Context, arg RemoveBrokenSongParams) error
 	RemoveBrokenSongs(ctx context.Context, uuids []uuid.UUID) error
 	RemoveSoundtrackDeletionDate(ctx context.Context, id int32) error
 	ResetSoundtrackIdSeq(ctx context.Context) (int64, error)
+	// Replaces the alternative names. A soundtrack without a cover is searched again with them.
+	SetAlternativeNames(ctx context.Context, arg SetAlternativeNamesParams) (Soundtrack, error)
+	SetCover(ctx context.Context, arg SetCoverParams) error
 	SetQueueCurrentSong(ctx context.Context, arg SetQueueCurrentSongParams) error
 	SetQueueLastFetched(ctx context.Context, arg SetQueueLastFetchedParams) error
 	SetSoundtrackDeletionDate(ctx context.Context) error