feat: save as .feedpak; discover and load both .feedpak and .sloppak (#553)

* feat: save songs as .feedpak; discover and load both .feedpak and .sloppak

The open song format was renamed sloppak -> feedpak (public spec lives in
the feedback-feedpak-spec repo), but the server still wrote and recognized
only `.sloppak`. The two are byte-identical on disk.

Read both suffixes everywhere songs are discovered, uploaded, and loaded;
writing the new `.feedpak` suffix is handled in the editor plugin repo. Keep
the internal `format` tag `sloppak` so existing feature gates (stems, drums,
keys) are untouched, matching the "internal rename not landed yet" stance.

- lib/sloppak.py: add FEEDPAK_EXT / SLOPPAK_EXT / SONG_EXTS; is_sloppak()
  now matches either suffix (covers all 7 callers).
- server.py: union scan glob over SONG_EXTS; widen loose-folder exclusion,
  settings DLC count, upload gate (_ALLOWED_SONG_EXTS) and zip-magic check;
  refresh user-facing messages to .feedpak.
- static: library format filter relabeled Sloppak -> Feedpak (value stays
  sloppak, matches both); badge text SLOPPAK -> FEEDPAK in v2 + v3;
  filename-suffix detection and upload drag-drop filter accept both.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: topkoa <topkoa@gmail.com>

* test: cover .feedpak/.sloppak dual-suffix support

Add tests/test_feedpak_extension.py pinning the four paths PR #553
widened so a refactor can't drop .sloppak back-compat or stop
accepting .feedpak:

- is_sloppak / SONG_EXTS suffix detection (file + dir form, case-insensitive)
- _background_scan discovery glob unions over both suffixes
- POST /api/songs/upload accepts both, rejects wrong suffix + non-zip
- save_settings DLC count includes both suffixes

19 tests, all passing; reuses the existing scan_module / TestClient /
isolate_logging fixtures.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Signed-off-by: topkoa <topkoa@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: byrongamatos <xasiklas@gmail.com>

static/index.html

@@ -120,7 +120,7 @@
                     <select id="lib-format" onchange="sortLibrary()"
                         class="bg-dark-700 border border-gray-800 rounded-xl px-3 py-2.5 text-sm text-gray-300 outline-none" title="Filter by format">
                         <option value="">All formats</option>
-                        <option value="sloppak">Sloppak</option>
+                        <option value="sloppak">Feedpak</option>
                         <option value="loose">Folder</option>
                     </select>
                     <!-- Tree controls -->