Split download from hosting; carry the token into enclosure URLsHEADmain

The machine that hosts a feed and the machine that downloads the audio do
not have to be the same, and usually shouldn't be: a datacentre IP is
treated very differently from a home connection by the larger video sites.
Downloading where the person actually is sidesteps that entirely, and means
the server needs no credentials for or route to the source.

- push/ingest: `podkit push` uploads a local library to a remote
  `serve --ingest`, asking for a manifest first so only the difference is
  sent. Uploads land via a temp file and rename, so an interrupted transfer
  cannot be picked up as a short episode. Ingest is off by default and
  refuses to start without a token.
- get: downloads a direct audio URL or every enclosure in an RSS feed,
  with no extractor involved. Numbers RSS items in reverse so a mirrored
  series plays in order rather than backwards.

Two bugs found by running it rather than by testing it:

- A tokened server produced enclosure URLs with no token. The feed loaded,
  listed every episode, and all of them 404'd in a real client — the whole
  point of the tool, broken in the default configuration. Feeds now carry
  the token; regression test asserts enclosures from a tokened feed
  actually resolve against a tokened server.
- Ingest ran filepath.Base before validating, so "../evil.mp3" was stored
  as "evil.mp3" — contained, but under a name the client never asked for,
  leaving push and the server disagreeing about the library. Names are now
  validated raw and rejected.

Also: identify with a real user-agent (libsyn 403s Go's default), write
feed.xml via rename so a polling client never reads a truncated feed, and
match ingested file permissions to downloaded ones.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018AwnALZL6bD4VBtATDVv3B

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