chore(deps): drop python-multipart -- nothing parses a multipart body now

Its own entry in requirements.txt said the download center's local-file upload
was "the only multipart endpoint in the app". Since that route reads a raw
body, the claim is false and the package is dead weight: no route declares
File/Form, and nothing calls `request.form()`.

Removed from requirements.txt and from its single pinned line in
requirements.lock. The lock is generated and normally regenerated wholesale,
but a full regeneration would bump every transitive dependency, which has no
business riding inside a security fix -- the next legitimate regeneration
reproduces exactly this state, since the package is gone from the source.

The `multipart` entry in logs.py's third-party level map goes with it: a level
for a logger that can no longer exist.

backend/requirements.lock

@@ -47,7 +47,6 @@ pydantic-settings==2.15.0
 pydantic==2.13.4
 pydantic_core==2.46.4
 python-dotenv==1.2.3
-python-multipart==0.0.32
 requests==2.34.2
 rich==15.0.0
 starlette==1.6.0