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/models.py

@@ -708,6 +708,12 @@ class MediaAsset(Base, TimestampMixin):
     container: Mapped[str | None] = mapped_column(String(16))
     vcodec: Mapped[str | None] = mapped_column(String(32))
     acodec: Mapped[str | None] = mapped_column(String(32))
+    # "video" | "audio" | "file" — the media-kind discriminator that drives which library actions
+    # apply (play/edit for media; download-only for a generic uploaded file). Null while an asset is
+    # still pending (kind is known once the file is probed at finalize). It is the single stored home
+    # of the invariant `downloads.mediakind.derive` used to compute — never re-derived in prose (an
+    # earlier ad-hoc `vcodec is None and acodec` check lived only in links.py; consolidated here).
+    media_kind: Mapped[str | None] = mapped_column(String(16))
     nfo_written: Mapped[bool] = mapped_column(Boolean, default=False, server_default="false")
     # Set once the GC has warned holders of the upcoming expiry, so it doesn't warn every run.
     gc_notified: Mapped[bool] = mapped_column(Boolean, default=False, server_default="false")