feat: auto-capitalize the first letter of transaction descriptions

Transactions/recurring templates entered with mixed casing ("траты на
продукты") are now capitalized on create/update via a Pydantic validator,
so the fix applies to every client, not just the web UI. Adds a one-off
backfill script for existing rows.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/core/text.py

@@ -0,0 +1,12 @@
+"""Shared text-formatting helpers."""
+
+
+def capitalize_first_letter(value: str) -> str:
+    """Uppercase the first character, leaving the rest of the string untouched.
+
+    Unlike str.capitalize()/str.title(), this never lowercases the rest of the
+    string — "iPhone 15" stays "iPhone 15", not "Iphone 15".
+    """
+    if not value:
+        return value
+    return value[0].upper() + value[1:]