Harden auth against a forwarded-header rate-limit bypass, and guard BYOK SSRF

The per-IP rate limits could be bypassed entirely: uvicorn ran with
--forwarded-allow-ips "*", which trusts the leftmost X-Forwarded-For value
(client-controlled), and Render forwards the inbound header rather than
stripping it. Rotating the header handed out a fresh rate-limit bucket per
request, so the login/register limit (10/5min) and guest-minting limit
(30/5min) were no throttle at all — unbounded password guessing and guest-row
creation. Confirmed live: fixed IP -> 429 after 10; rotating spoofed header ->
no 429 across 14 attempts.

Two-layer fix:
- limits._client_ip now derives the client IP from the hop the trusted edge
  appends (rightmost of X-Forwarded-For), which a client can't spoof past;
  tunable via AIDND_TRUSTED_PROXY_HOPS. Dropped --forwarded-allow-ips "*".
- New per-account login throttle (email-keyed, 8 fails / 15 min, cleared on
  success): stops distributed guessing against one account that a per-IP limit
  can't, since it can't be diluted across many source addresses.

Also close an SSRF on the BYOK endpoint_url (hosted mode only): the connection
test and turn/chat streams now refuse a URL that resolves to a non-public
address (private/loopback/link-local metadata/reserved), checked at request
time so it resists a DNS record flipping to a private IP. No-op locally, where
reaching localhost Ollama is intended.

Tests: test_ratelimit_hardening.py (8), test_netguard.py (13). 172 pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015CYEJKobJ2Re4Dv7qUoSA7

backend/app/routers/auth.py

@@ -105,13 +105,18 @@ def login(
         raise HTTPException(400, "Accounts are disabled in local mode.")
     limits.rate_limit("auth", request)
     email = payload.email.strip().lower()
+    # Per-account throttle: stops distributed guessing against one email even
+    # when the per-IP limit above is diluted across many source addresses.
+    limits.check_login_allowed(email)
     user = db.query(models.User).filter(models.User.email == email).first()
     if (
         user is None
         or not user.password_hash
         or not security.verify_password(payload.password, user.password_hash)
     ):
+        limits.note_login_failure(email)
         raise HTTPException(401, "Incorrect email or password.")
+    limits.note_login_success(email)
     _set_session_cookie(response, user.id)
     return me_payload(user, db)