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>

lib/sloppak.py

@@ -29,6 +29,12 @@ log = logging.getLogger("slopsmith.lib.sloppak")
 # (additive/MINOR compatibility); writers stamp this.
 FEEDPAK_VERSION = "1.2.0"
 
+# Package suffixes. The format is byte-identical regardless of suffix; `.feedpak`
+# is the current write extension, `.sloppak` the legacy one we still read.
+FEEDPAK_EXT = ".feedpak"
+SLOPPAK_EXT = ".sloppak"
+SONG_EXTS = (FEEDPAK_EXT, SLOPPAK_EXT)  # accepted on read/discovery
+
 import yaml
 
 from safepath import safe_join
@@ -48,8 +54,12 @@ import notation as notation_mod
 # ── Format detection ──────────────────────────────────────────────────────────
 
 def is_sloppak(path: Path) -> bool:
-    """True if path looks like a sloppak (zip file or directory)."""
-    return path.name.lower().endswith(".sloppak")
+    """True if path looks like a song package (zip file or directory).
+
+    Accepts both the current `.feedpak` suffix and the legacy `.sloppak` one —
+    same on-disk format, either form.
+    """
+    return path.name.lower().endswith(SONG_EXTS)
 
 
 # ── Source resolution (zip unpack cache + directory passthrough) ──────────────