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/alembic/versions/0076_media_asset_kind.py

@@ -0,0 +1,41 @@
+"""media_assets.media_kind (E7 S5a: the video|audio|file discriminator for the media library)
+
+Adds a stored media-kind to each asset so the library can gate its actions on it (play/edit for
+media, download-only for a generic uploaded file) instead of re-deriving from codecs in prose. The
+column is the single home of the invariant `downloads.mediakind.derive` computes at finalize.
+
+Backfill for existing rows: a stored video codec -> 'video', an audio-only codec -> 'audio', an
+editor derivative (source_kind='edit', which carries no codecs but is always a video clip) ->
+'video'. Rows still pending/errored (no codecs, not an edit) stay NULL — kind is set when they
+finalize. Empty `media_kind` never reaches the UI: only `ready`/`done` rows render actions, and
+those are all backfilled here.
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0076_media_asset_kind"
+down_revision: Union[str, None] = "0075_playlist_sync_baseline"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("media_assets", sa.Column("media_kind", sa.String(length=16), nullable=True))
+    op.execute(
+        """
+        UPDATE media_assets
+           SET media_kind = CASE
+               WHEN vcodec IS NOT NULL THEN 'video'
+               WHEN acodec IS NOT NULL THEN 'audio'
+               WHEN source_kind = 'edit' THEN 'video'
+               ELSE NULL
+           END
+        """
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("media_assets", "media_kind")