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/config/config_test.go

@@ -33,6 +33,18 @@ func TestLoadDefaults(t *testing.T) {
 	assert.Empty(t, cfg.TrustedProxies)
 	assert.Empty(t, cfg.CORSAllowedOrigins)
 	assert.Equal(t, DefaultMetricsPort, cfg.MetricsPort)
+	assert.Empty(t, cfg.CoversPath, "covers are off by default")
+	assert.Empty(t, cfg.SteamGridDBAPIKey)
+}
+
+func TestLoadCovers(t *testing.T) {
+	values := validEnv()
+	values["COVERS_PATH"] = "/covers"
+	values["STEAMGRIDDB_API_KEY"] = "key"
+	cfg, err := load(env(values))
+	require.NoError(t, err)
+	assert.Equal(t, "/covers", cfg.CoversPath)
+	assert.Equal(t, "key", cfg.SteamGridDBAPIKey)
 }
 
 func TestLoadMetricsPort(t *testing.T) {