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/netguard.py

@@ -1,14 +1,15 @@
 """SSRF guard for the one place the server makes an outbound request to a
 user-supplied address: the BYOK `endpoint_url` (connection test + turns/chat).
 
-Without this, a hosted user could point endpoint_url at an internal service or
-the cloud metadata endpoint (169.254.169.254) and have the server fetch it —
-the connection test even echoes part of the response back. We refuse any URL
-that resolves to a non-public address.
+Without this guard, a hosted user could point `endpoint_url` at an internal
+service or at the cloud metadata endpoint, 169.254.169.254, and have the server
+fetch it. The connection test even returns part of the response. The guard
+therefore refuses any URL that resolves to a non-public address.
 
-No-op in local mode: a local install talking to http://localhost:11434 (Ollama)
-is the normal, intended case — the guard only applies to the hosted, multi-user
-deployment where the endpoint comes from an untrusted visitor.
+The guard does nothing in local mode. A local install talking to
+http://localhost:11434, which is Ollama, is the intended case. The guard applies
+only to a hosted, multi-user deployment, where the endpoint comes from an
+untrusted visitor.
 """
 
 import ipaddress