Support extra CA bundle for private-CA LLM providers (#769)

Adding GigaChat (Sber) or an on-premise enterprise LLM gateway as a
model endpoint fails on first probe with

    CERTIFICATE_VERIFY_FAILED: self-signed certificate in certificate
    chain (_ssl.c:1000)

because their TLS chain is signed by a private root CA (Russian Trusted
Root CA for GigaChat; corporate CA for on-prem) that isn't part of the
default system / certifi trust store. The endpoint shows offline in
the picker even though the URL and API key are correct (issue #722).

The right fix is to extend the trust store, not to weaken verification.
This change:

- src/tls_overrides.py: new module that resolves an opt-in env var
  LLM_CA_BUNDLE at import time, builds a shared SSLContext via
  ssl.create_default_context() (so the system / certifi bundle is
  loaded first) and layers the operator's PEM on top with
  load_verify_locations(). Exposes llm_verify() returning a value
  suitable for httpx `verify=`. Defaults to True (httpx built-in
  trust) when the env var is unset, when the file is missing, or
  when the PEM fails to load — verification is never silently
  disabled, the warning is logged and we fall back to the safe path.

- src/llm_core.py: thread llm_verify() into the shared AsyncClient
  used by stream_llm / streaming completions.

- routes/model_routes.py: thread llm_verify() into the five httpx.get
  call sites in _probe_endpoint / _ping_endpoint so adding a
  private-CA endpoint goes green on the very first probe and the
  picker stops showing it offline.

- .env.example: document LLM_CA_BUNDLE with the GigaChat case as the
  concrete example.

Deliberately NOT included: a verify=False knob (global or per-host).
Disabling verification exposes the affected endpoint to MITM, and the
operator-supplied bundle is the correct fix for legitimate private-CA
providers — so the only switch in this PR is the safe one.

Closes #722.

src/llm_core.py

@@ -129,7 +129,10 @@ def _get_http_client() -> httpx.AsyncClient:
     """Return process-wide AsyncClient. Per-request timeout is passed at call time."""
     global _http_client
     if _http_client is None or _http_client.is_closed:
-        _http_client = httpx.AsyncClient(limits=_http_limits, http2=False)
+        from src.tls_overrides import llm_verify
+        _http_client = httpx.AsyncClient(
+            limits=_http_limits, http2=False, verify=llm_verify(),
+        )
     return _http_client
 
 def _get_cached_response(cache_key: str) -> Optional[str]: