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/pipeline.py

@@ -15,7 +15,7 @@ from app.config import settings
 from app.db import SessionLocal
 from app.downloads import edit as editmod
 from app.downloads import errors as dlerrors
-from app.downloads import adapters, credentials, formats, service, storage
+from app.downloads import adapters, credentials, formats, mediakind, service, storage
 from app.downloads.jobdb import (
     _edit_staging_dir,
     _job_status,
@@ -484,6 +484,7 @@ def _download(
                 asset.acodec = (
                     info.get("acodec") if info.get("acodec") not in (None, "none") else None
                 )
+                asset.media_kind = mediakind.derive(asset.vcodec, asset.acodec)
                 asset.title = meta.title
                 asset.uploader = meta.uploader
                 asset.uploader_url = adapter.uploader_url(info)