Rewrite Python comments in Google developer documentation style (#12)

* Rewrite comments in Google developer documentation style

Rewrite the comments and docstrings across the backend core modules so they
read plainly. The previous prose was accurate but dense and figurative, which
made it slow to skim.

Applies the Google developer documentation style guide: short sentences, active
voice, present tense, American spelling, and no metaphors, idioms, or
rhetorical asides. Replaces em-dash chains with separate sentences.

backend/app/routers/chat.py

@@ -1,14 +1,15 @@
-"""AI Chat — a plain scratchpad for talking to a model directly.
+"""AI Chat: a plain scratchpad for talking to a model directly.
 
-Power users only (the AIDND_POWER_USERS email allowlist). Deliberately thin:
-no story context, no scripts, no world state, and nothing persisted — the
-conversation lives in the browser and is posted up whole on each turn. It
-exists to poke at models, prompts and endpoints without starting an adventure.
+Power users reach it, which means the `AIDND_POWER_USERS` email allowlist. It is
+deliberately thin. It adds no story context, no scripts, and no world state, and
+it persists nothing. The conversation lives in the browser and is posted in full
+on each turn. It exists for testing models, prompts, and endpoints without
+starting an adventure.
 
-Model choice is free-form when the user brought their own API key. On the
-shared demo key it stays pinned to the AIDND_DEMO_MODELS whitelist, exactly as
-turns are: the server funds that key, so it must not be able to reach paid
-models by way of this page.
+Model choice is free when the user brought their own API key. On the shared demo
+key the model stays pinned to the `AIDND_DEMO_MODELS` allowlist, exactly as it is
+for turns. The server funds that key, so this page must not let it reach paid
+models.
 """
 
 from fastapi import APIRouter, Depends, HTTPException, Request
@@ -41,10 +42,13 @@ PowerUser = Depends(power_user)
 def _resolve_model(
     settings: models.Settings, requested: str | None
 ) -> tuple[auth.ProviderConfig, str | None]:
-    """Provider config for this chat, plus a note when the requested model was
-    not honoured. The pinning rule itself lives in resolve_provider_config —
-    this only reports the substitution it made, so there is exactly one place
-    that decides what the demo key is allowed to talk to."""
+    """Returns the provider config for this chat, plus a note when the requested
+    model was not used.
+
+    The pinning rule lives in `resolve_provider_config`. This function only
+    reports the substitution that call made, so one place decides what the demo
+    key may talk to.
+    """
     cfg = auth.resolve_provider_config(settings, model_override=requested)
     wanted = (requested or "").strip()
     if wanted and wanted != cfg.model:
@@ -60,9 +64,12 @@ async def chat_config(
     db: Session = Depends(get_db),
     user: models.User = PowerUser,
 ):
-    """What this page can talk to: the resolved endpoint/model, whether model
-    choice is pinned to the demo whitelist, and the endpoint's model listing
-    (best effort — an unreachable endpoint just yields an empty list)."""
+    """Returns what this page can talk to.
+
+    The response holds the resolved endpoint and model, whether model choice is
+    pinned to the demo allowlist, and the endpoint's model listing. The listing
+    is best effort, and an unreachable endpoint returns an empty list.
+    """
     settings = get_settings(db, user)
     cfg = auth.resolve_provider_config(settings)
     listing = await list_endpoint_models(cfg)
@@ -82,8 +89,11 @@ async def chat_config(
 
 async def run_chat(cfg: auth.ProviderConfig, settings: models.Settings, payload: schemas.ChatRequest,
                    note: str | None, db: Session, user: models.User):
-    """SSE generator mirroring the turn stream's event shape: reasoning/chunk
-    while generating, then done — so the frontend reuses the same plumbing."""
+    """Streams the reply as SSE, using the turn stream's event shape.
+
+    The generator emits `reasoning` and `chunk` events while generating and then
+    a `done` event, so the frontend reuses the same code.
+    """
     if note:
         yield sse({"type": "note", "detail": note})
     provider = OpenAICompatibleProvider(