Add power-user AI Chat page; centralize demo-key model pinning

AI Chat is a plain scratchpad for talking to a model directly — no story
context, scripts or world state — for poking at models, prompts and endpoints
without starting an adventure. Power users only: the router 404s (rather than
403s) for everyone else and the nav link is hidden. The conversation lives in
localStorage, so there's no new table or migration.

is_power_user() now also returns True in local mode: it's the operator's own
machine and their own key, the same reasoning that makes the provider debug log
local-only.

Alongside that, the rule keeping the shared demo key off paid models now lives
in exactly one place. It had been duplicated into the chat router, which is how
one copy eventually drifts:

- resolve_provider_config() takes an optional model_override and is the only
  place the whitelist is applied, so turns, AI Chat and the connection test all
  inherit it. An override is a per-request preference, never a grant.
- ProviderConfig.__post_init__ refuses to exist when api_key is the demo key
  and the model isn't whitelisted. It keys on the key itself rather than the
  using_demo flag, so a mislabelled config can't slip past, and it raises so a
  future path that bypasses the resolver fails loudly instead of billing.
- The demo branch still pins endpoint_url too — a user-controlled endpoint
  would leak the key itself, which is worse than spending it.

Provider gained chat(messages, ...) beside generate(), both delegating to a
shared _stream(url, body); completion-mode endpoints get the messages flattened
into a labelled transcript. Settings' /models fetch moved to
list_endpoint_models() and is shared with /api/chat/config.

Tests: 10 new in tests/test_chat.py (70 total). These deliberately do not stub
resolve_provider_config — the point is to exercise the real BYOK-vs-demo
decision and assert on what the provider actually received: off-whitelist
override pinned, off-whitelist Settings.model pinned, redirected endpoint
pinned, BYOK passed through untouched.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014FGY1yvzSeKgTtRfeVtDmx

backend/app/routers/settings.py

@@ -62,18 +62,9 @@ def update_settings(
     return settings
 
 
-@router.post("/test")
-async def test_connection(
-    request: Request,
-    db: Session = Depends(get_db),
-    user: models.User = Depends(auth.get_current_user),
-):
-    """Hit the endpoint's /models listing as a cheap connectivity check.
-    Tests whatever the turn engine would actually use — including the shared
-    demo endpoint when the user has no key of their own."""
-    limits.rate_limit("connection-test", request, user)
-    settings = get_settings(db, user)
-    cfg = auth.resolve_provider_config(settings)
+async def list_endpoint_models(cfg: auth.ProviderConfig) -> dict:
+    """GET the endpoint's /models listing. Doubles as a connectivity check, so
+    failures come back as {"ok": False, "detail": ...} rather than raising."""
     url = cfg.endpoint_url.rstrip("/") + "/models"
     headers = {}
     if cfg.api_key:
@@ -94,3 +85,16 @@ async def test_connection(
     except (ValueError, AttributeError, TypeError):
         pass  # non-JSON or unexpected shape — connectivity is still confirmed
     return {"ok": True, "models": models_available}
+
+
+@router.post("/test")
+async def test_connection(
+    request: Request,
+    db: Session = Depends(get_db),
+    user: models.User = Depends(auth.get_current_user),
+):
+    """Cheap connectivity check against whatever the turn engine would actually
+    use — including the shared demo endpoint when the user has no key."""
+    limits.rate_limit("connection-test", request, user)
+    settings = get_settings(db, user)
+    return await list_endpoint_models(auth.resolve_provider_config(settings))