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/utils.py

@@ -1,6 +1,7 @@
 """Shared utility helpers."""
 
 
+# Define function escape_like
 def escape_like(value: str) -> str:
     """Escape SQL LIKE wildcard characters (``%`` and ``_``).
 
@@ -13,9 +14,13 @@ def escape_like(value: str) -> str:
         from app.utils import escape_like
         query.filter(Model.name.ilike(f"%{escape_like(term)}%"))
     """
+    # Return (
     return (
         value
+        # Chain .replace() call
         .replace("\\", "\\\\")
+        # Chain .replace() call
         .replace("%", "\\%")
+        # Chain .replace() call
         .replace("_", "\\_")
     )