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/auth.py

@@ -81,19 +81,50 @@ def demo_enabled() -> bool:
 @dataclass
 class ProviderConfig:
     """What the turn engine should actually connect with, after the
-    BYOK-vs-demo decision."""
+    BYOK-vs-demo decision. Build these with resolve_provider_config()."""
 
     endpoint_url: str
     api_key: str
     model: str
     using_demo: bool
 
+    def __post_init__(self) -> None:
+        # Belt and braces around the shared demo key. resolve_provider_config()
+        # already pins the model, but this makes it a property of the config
+        # object itself: however it was built, and by whichever caller, the
+        # server-funded key can never be paired with an off-whitelist (i.e.
+        # possibly paid) model. Unreachable by design — a 500 here means a new
+        # code path tried to bypass the pinning, which is worth failing loudly
+        # rather than silently billing.
+        if DEMO_API_KEY and self.api_key == DEMO_API_KEY and self.model not in DEMO_MODELS:
+            raise ValueError(
+                f"Refusing to use the shared demo key with non-whitelisted model {self.model!r}"
+            )
 
-def resolve_provider_config(settings: models.Settings) -> ProviderConfig:
+
+def resolve_provider_config(
+    settings: models.Settings, *, model_override: str | None = None
+) -> ProviderConfig:
+    """BYOK when the user has their own key, the shared demo key otherwise.
+
+    THE security-relevant branch is the demo one, and it is the only place the
+    whitelist rule lives — every caller must come through here rather than
+    building a ProviderConfig itself. On the demo key:
+
+    - the model is pinned to DEMO_MODELS, so a caller-supplied override (the AI
+      Chat page) or a hand-edited Settings row cannot aim a server-funded key at
+      a paid model; anything unrecognised falls back to DEMO_MODELS[0];
+    - the endpoint is pinned to DEMO_ENDPOINT_URL, so the key itself can't be
+      redirected to a URL the user controls and harvested.
+
+    `model_override` is a per-request preference (never a grant): it's honoured
+    verbatim under BYOK, and only if whitelisted on the demo key.
+    """
     key = settings.api_key_plain
+    requested = (model_override or "").strip() or settings.model
     if key or not demo_enabled():
-        return ProviderConfig(settings.endpoint_url, key, settings.model, False)
-    model = settings.model if settings.model in DEMO_MODELS else DEMO_MODELS[0]
+        return ProviderConfig(settings.endpoint_url, key, requested, False)
+    model = requested if requested in DEMO_MODELS else DEMO_MODELS[0]
     return ProviderConfig(DEMO_ENDPOINT_URL, DEMO_API_KEY, model, True)
 
 
@@ -102,7 +133,12 @@ def _today() -> str:
 
 
 def is_power_user(user: models.User) -> bool:
-    """Trusted testers (email allowlist) bypass the demo turn cap."""
+    """Trusted testers: unmetered demo turns, plus tooling that isn't part of
+    the game (the AI Chat scratchpad). Local installs are always trusted — it's
+    the operator's own machine and their own API key, same reasoning as the
+    provider debug log being local-only."""
+    if not MULTI_USER:
+        return True
     return bool(user.email) and user.email.lower() in POWER_USERS