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/queries/statistics.sql

@@ -5,6 +5,8 @@ SELECT
     g.uuid as soundtrack_uuid,
     g.soundtrack_name,
     g.times_played as soundtrack_played,
+    g.alternative_names,
+    g.cover_modified,
     g.last_played as soundtrack_last_played,
     COALESCE(
         json_agg(
@@ -33,6 +35,8 @@ SELECT
     g.uuid as soundtrack_uuid,
     g.soundtrack_name,
     g.times_played as soundtrack_played,
+    g.alternative_names,
+    g.cover_modified,
     g.last_played as soundtrack_last_played,
     COALESCE(
         json_agg(
@@ -95,6 +99,8 @@ SELECT
     g.uuid as soundtrack_uuid,
     g.soundtrack_name,
     g.times_played as soundtrack_played,
+    g.alternative_names,
+    g.cover_modified,
     g.added,
     COALESCE(
         json_agg(
@@ -121,6 +127,8 @@ SELECT
     g.uuid as soundtrack_uuid,
     g.soundtrack_name,
     g.times_played as soundtrack_played,
+    g.alternative_names,
+    g.cover_modified,
     g.last_played as soundtrack_last_played,
     COALESCE(
         json_agg(
@@ -148,6 +156,8 @@ SELECT
     g.uuid as soundtrack_uuid,
     g.soundtrack_name,
     g.times_played as soundtrack_played,
+    g.alternative_names,
+    g.cover_modified,
     g.last_played as soundtrack_last_played,
     COALESCE(
         json_agg(
@@ -188,6 +198,8 @@ SELECT
     g.uuid as soundtrack_uuid,
     g.soundtrack_name,
     g.times_played as soundtrack_played,
+    g.alternative_names,
+    g.cover_modified,
     g.last_played as soundtrack_last_played,
     COALESCE(
         json_agg(