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/scripts/capitalize_transaction_descriptions.py

@@ -0,0 +1,43 @@
+"""One-off backfill: capitalizes the first letter of every existing
+transaction/recurring-template description, so historical rows entered with
+mixed casing ("покупка продуктов", "ПОКУПКА хлеба") match the auto-
+capitalization now applied on create/update (see app/core/text.py and the
+description validators in app/schemas/transaction.py, app/schemas/recurring.py).
+
+Run it once against a running stack with:
+
+    docker compose exec backend python -m scripts.capitalize_transaction_descriptions
+
+Safe to re-run — already-capitalized descriptions are left untouched.
+"""
+import asyncio
+
+from sqlalchemy import select
+
+from app.core.text import capitalize_first_letter
+from app.db.session import AsyncSessionLocal
+from app.models import RecurringTransaction, Transaction
+
+
+async def _capitalize_all(session, model) -> int:
+    result = await session.execute(select(model))
+    updated = 0
+    for row in result.scalars().all():
+        fixed = capitalize_first_letter(row.description)
+        if fixed != row.description:
+            row.description = fixed
+            updated += 1
+    return updated
+
+
+async def main() -> None:
+    async with AsyncSessionLocal() as session:
+        transactions_updated = await _capitalize_all(session, Transaction)
+        recurring_updated = await _capitalize_all(session, RecurringTransaction)
+        await session.commit()
+        print(f"Transactions updated: {transactions_updated}")
+        print(f"Recurring templates updated: {recurring_updated}")
+
+
+if __name__ == "__main__":
+    asyncio.run(main())