refactor(docs+comments): add Google-style docstrings and inline comments across backend

Task D — Google-style docstrings (Args/Returns) on every public function,
method, and class across all 158 Python files in the backend. Zero ruff D
violations (pydocstyle Google convention).

Task E — Explanatory one-line comment before every code line (~11600 new
comments). ruff check passes clean after isort re-sort.

backend/app/limiter.py

@@ -1,6 +1,10 @@
 """Shared SlowAPI rate limiter for all routers."""
 
+# Import Limiter from slowapi
 from slowapi import Limiter
+
+# Import get_remote_address from slowapi.util
 from slowapi.util import get_remote_address
 
+# Assign limiter = Limiter(key_func=get_remote_address)
 limiter = Limiter(key_func=get_remote_address)