feat(downloads): local file upload ingest — media library backend (E7 S5a)

Add the non-yt-dlp ingest path so a user-uploaded file joins the same download
library and inherits its function set. Any file type is accepted: media
(video/audio) gets the full set (metadata, share, play/edit), a generic file
(pdf/zip/docx/…) is download-only.

- MediaAsset.media_kind ("video"|"audio"|"file") — the stored discriminator that
  gates library actions. Set at every finalize (download probe, editor carries the
  source's kind, upload probe); consolidates the ad-hoc audio-detect that lived only
  in links.py into one invariant (downloads.mediakind.derive). Migration 0076 adds
  the column + backfills existing rows.
- downloads/upload.py: ingest_upload probes the file (ffprobe → codecs/dims/duration),
  places it in a per-user .uploads tree, and creates an already-ready asset + done job
  (the same cache-hit shortcut enqueue uses). Uploads never expire (irreplaceable).
- POST /api/downloads/upload: streaming multipart with the per-file cap
  (download_upload_max_bytes, default 500 MiB) enforced on actual bytes read; quota
  charges the known size up front (quota.check_upload).
- Security: a generic file is always served force-download (attachment +
  application/octet-stream) on both the in-app and public endpoints, so uploaded
  HTML/SVG can't execute inline. Edit + redownload reject non-media / upload sources.
- python-multipart added (first multipart endpoint in the app).

backend/app/downloads/storage.py

@@ -147,6 +147,28 @@ def edit_rel_path(user_id: int, asset_id: int, ext: str) -> str:
     return f".edits/{user_id}/clip_{asset_id}.{ext}"
 
 
+def upload_rel_path(user_id: int, asset_id: int, filename: str) -> str:
+    """On-disk path (relative to DOWNLOAD_ROOT) for a locally-uploaded file.
+
+    Kept in a per-user `.uploads/` tree — device-downloadable via the file endpoint, but NOT part of
+    the Plex library (an upload is arbitrary user content, not a canonical episode; no .nfo/poster
+    episode sidecars). The name is system-decided and bound to the asset id (`_a{id}` keeps two
+    same-named uploads apart); the original filename is display metadata only. The extension is
+    preserved (shell-safe, bounded) so the served file + its Content-Disposition carry a correct
+    suffix; an extensionless upload stays extensionless."""
+    stem = sanitize(Path(filename).stem, 80)
+    ext = _UNSAFE.sub("", Path(filename).suffix.lstrip(".")).lower()[:16]
+    suffix = f".{ext}" if ext else ""
+    return f".uploads/{user_id}/{stem}_a{asset_id}{suffix}"
+
+
+def upload_ext(filename: str) -> str | None:
+    """The stored `container` for an uploaded file: its shell-safe lowercased extension, or None when
+    the upload has no extension (a bare name)."""
+    ext = _UNSAFE.sub("", Path(filename).suffix.lstrip(".")).lower()[:16]
+    return ext or None
+
+
 def download_filename(display_name: str, container: str | None, path: Path) -> str:
     """The Content-Disposition filename for a served download: the cleaned display name carrying a single
     correct extension — the asset container, else the file's own suffix — without doubling it when the