Accept feedpak name/extension as additive alias for sloppak

Phase 1 of deprecating the format name sloppak -> feedpak. Purely additive
and fully back-compat: nothing sloppak stops working, and no on-disk/runtime
artifacts (cache dir, config, SLOPSMITH_* env) are touched.

- lib/sloppak.py: detection accepts both `.feedpak` and `.sloppak`
  (new PACK_SUFFIXES / is_pack / is_feedpak; is_sloppak kept as a deprecated
  alias that now matches both). Read the optional top-level `feedpak_version`
  manifest key onto LoadedSloppak and into extract_meta(). Add LoadedFeedpak
  class alias and a deprecation note in the module docstring.
- lib/feedpak.py (new): canonical module name; re-exports the sloppak public
  API unchanged so new code can `import feedpak`.
- server.py: accept `.feedpak` uploads (_ALLOWED_SONG_EXTS + message); add a
  `get_feedpak_cache_dir` plugin-context key alongside the legacy
  `get_sloppak_cache_dir` (same dir); tolerate a `feedpak` value in the
  library format filter.
- static/: accept `.feedpak` in the upload picker (index.html accept=, app.js
  client-side filter).
- tests/test_feedpak_alias.py: both extensions detected + loaded via both
  module names, feedpak_version read, legacy aliases intact.

The emitted `format` library tag stays "sloppak" for both extensions this
phase (flipping it to "feedpak" with badge/gating updates is a deliberate
frontend-touching follow-up). Format spec: got-feedback/feedback-feedpak-spec.

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

static/index.html

@@ -71,7 +71,7 @@
 
     <!-- Hidden file input shared by the navbar "Upload" link. Kept at body
          level so it stays reachable regardless of which screen is active. -->
-    <input type="file" id="upload-songs-file" accept=".sloppak" multiple class="hidden" onchange="uploadSongs(this.files); this.value=''">
+    <input type="file" id="upload-songs-file" accept=".feedpak,.sloppak" multiple class="hidden" onchange="uploadSongs(this.files); this.value=''">
 
     <!-- ══ HOME (Hero + Library) ══════════════════════════════════════════ -->
     <div id="home" class="screen active">