Phase 0.1: Remove backend deprecated code

- Delete routes: counter.py, sum.py
- Delete jobs.py and worker.py
- Delete tests: test_counter.py, test_jobs.py
- Update audit.py: keep only price-history endpoints
- Update models.py: remove VIEW_COUNTER, INCREMENT_COUNTER, USE_SUM permissions
- Update models.py: remove Counter, SumRecord, CounterRecord, RandomNumberOutcome models
- Update schemas.py: remove sum/counter related schemas
- Update main.py: remove deleted router imports
- Update test_permissions.py: remove tests for deprecated features
- Update test_price_history.py: remove worker-related tests
- Update conftest.py: remove mock_enqueue_job fixture
- Update auth.py: fix example in docstring

backend/auth.py

@@ -90,9 +90,9 @@ def require_permission(*required_permissions: Permission):
     Dependency factory that checks if user has ALL required permissions.
 
     Usage:
-        @app.get("/api/counter")
-        async def get_counter(
-            user: User = Depends(require_permission(Permission.VIEW_COUNTER))
+        @app.get("/api/profile")
+        async def get_profile(
+            user: User = Depends(require_permission(Permission.MANAGE_OWN_PROFILE))
         ):
             ...
     """