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/routes/public.py

@@ -15,7 +15,7 @@ from sqlalchemy.orm import Session
 from app.config import settings
 from app.db import get_db
 from app.downloads import links as linksmod
-from app.downloads import storage
+from app.downloads import mediakind, storage
 from app.models import DownloadJob, DownloadLink, MediaAsset
 from app.ratelimit import RateLimiter
 from app.security import verify_password
@@ -122,8 +122,18 @@ def watch_file(token: str, g: str | None = None, db: Session = Depends(get_db)):
         raise HTTPException(status_code=404, detail="This video is no longer available.")
 
     filename = storage.download_filename(asset.title or "video", asset.container, path)
+    # A generic uploaded file (media_kind='file') is ALWAYS a force-download attachment with a
+    # neutral type — never served inline — so arbitrary uploaded HTML/SVG can't execute on this
+    # login-free public origin. Media follows the link's own inline/attachment choice so it can play
+    # in the watch page. Both are range-aware (FileResponse → 206) for seeking/resume.
+    if asset.media_kind == mediakind.FILE:
+        return FileResponse(
+            path,
+            filename=filename,
+            media_type="application/octet-stream",
+            content_disposition_type="attachment",
+        )
     disposition = "attachment" if link.allow_download else "inline"
-    # Starlette's FileResponse honours the Range header (206) for seeking/scrubbing.
     return FileResponse(path, filename=filename, content_disposition_type=disposition)