Believe forwarded client addresses only from trusted proxies - #16

The problem

The anonymous rate limit and the upload audit trail are both keyed on the client
address. An address the client can choose is an address that resets the budget
and writes fiction into the audit trail.

Two layers were each trusting X-Forwarded-For on their own:

1. app/fingerprint.py read the header whenever TRUST_FORWARDED_FOR was set,
   and took split(",")[0] — the client-controlled end of the chain, since
   proxies append.
2. The container ran uvicorn with --proxy-headers --forwarded-allow-ips "*",
   which rewrote the peer address from the same header for any caller,
   before the application saw it.

Consequence of (2): TRUST_FORWARDED_FOR=false was not an off switch. With
it set to false, a forged header still landed in the audit trail, because the
server had already rewritten the peer address.

Is it reachable?

Tested rather than assumed.

• From the internet: no. The app publishes no host port, and the front
  replaces X-Forwarded-For on the way through. A forged header sent to
  https://share.beckham.ai/api/anon-upload was discarded; the row recorded the
  real address.
• From the container network: yes. Four other containers share idp_proxy
  with this app. A POST straight to beckham-share-app:8000 carrying
  X-Forwarded-For: 198.51.100.7 was recorded verbatim in upload_events.

So the exposure is "any container on the shared proxy network", not "anyone on
the internet" — but that is exactly the layer the in-app control exists to
backstop.

The change

uvicorn now reports the socket peer, and the application makes the decision in
one place:

• New TRUSTED_PROXIES setting — comma-separated addresses, CIDR ranges, or
  hostnames, resolved at runtime because Docker assigns container addresses.
  Defaults to idp-caddy, which resolves to 172.23.0.5 from inside the app
  container.
• Forwarded headers are honoured only when the request arrived from one of
  those peers. Everything else is attributed to the address it came from,
  whatever headers it carries.
• The chain is read right to left: trusted hops are skipped, and the first
  untrusted address is the closest one a trusted proxy actually observed.
• TRUST_FORWARDED_FOR is replaced by TRUSTED_PROXIES. Nothing set the old
  variable, so there is nothing to migrate.

What is deliberately not changed

The browser fingerprint stays client-supplied. It is OR-ed with the address,
not substituted for it — app/ratelimit.py counts rows matching either key,
so rotating the fingerprint cannot buy a fresh budget while the address key
still matches. Its job is the reverse: catching one device that rotates
addresses. Folding the address into it would destroy exactly that property.
Now stated explicitly in docs/SECURITY.md so the distinction reads as a
decision rather than an oversight.

Verification

Staging container, real requests:

Plus 8 unit tests over client_ip() and a regression test that rotating the
header does not reset the anonymous budget. Backend suite: 33 tests pass.

Deploy note

This changes the container command, so it needs a rebuild rather than a restart.
docs/SECURITY.md §3 and §6 and docs/CONFIGURATION.md are updated to match.