Phase 10: Per-User Rate Limits — messages + tokens, quota UI, admin usage

Backend:
- max_ai_messages_per_day + max_ai_tokens_per_day on User model (nullable, override)
- Migration 008: add columns + seed default settings (100 msgs, 500K tokens)
- usage_service: count today's messages + tokens, check quota, get limits
- GET /chats/quota returns usage vs limits + reset time
- POST /chats/{id}/messages checks quota before streaming (429 if exceeded)
- Admin user schemas expose both limit fields
- GET /admin/usage returns per-user daily message + token counts
- admin_user_service allows updating both limit fields

Frontend:
- Chat header shows "X/Y messages · XK/YK tokens" with red highlight at limit
- Quota refreshes every 30s via TanStack Query
- Admin usage page with table: user, messages today, tokens today
- Route + sidebar entry for admin usage
- English + Russian translations

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/alembic/versions/008_add_user_ai_rate_limit.py

@@ -0,0 +1,34 @@
+"""Add AI rate limit columns to users, seed default settings
+
+Revision ID: 008
+Revises: 007
+Create Date: 2026-03-19
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = "008"
+down_revision: Union[str, None] = "007"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("users", sa.Column("max_ai_messages_per_day", sa.Integer, nullable=True))
+    op.add_column("users", sa.Column("max_ai_tokens_per_day", sa.Integer, nullable=True))
+
+    op.execute("""
+        INSERT INTO settings (id, key, value) VALUES
+        (gen_random_uuid(), 'default_max_ai_messages_per_day', '100'),
+        (gen_random_uuid(), 'default_max_ai_tokens_per_day', '500000')
+        ON CONFLICT (key) DO NOTHING
+    """)
+
+
+def downgrade() -> None:
+    op.drop_column("users", "max_ai_tokens_per_day")
+    op.drop_column("users", "max_ai_messages_per_day")
+    op.execute("DELETE FROM settings WHERE key IN ('default_max_ai_messages_per_day', 'default_max_ai_tokens_per_day')")