feat(schemas): T30 add Pydantic statistics schemas

Add comprehensive Pydantic schemas for statistics management:
- UsageStatsCreate: input validation for creating usage stats
- UsageStatsResponse: orm_mode response schema
- StatsSummary: aggregated statistics with totals and averages
- StatsByModel: per-model breakdown with percentages
- StatsByDate: daily usage aggregation
- DashboardResponse: complete dashboard data structure

All schemas use Decimal for cost precision and proper validation.

Test: 16 unit tests, 100% coverage on stats.py

src/openrouter_monitor/schemas/__init__.py

@@ -12,6 +12,14 @@ from openrouter_monitor.schemas.auth import (
     UserRegister,
     UserResponse,
 )
+from openrouter_monitor.schemas.stats import (
+    DashboardResponse,
+    StatsByDate,
+    StatsByModel,
+    StatsSummary,
+    UsageStatsCreate,
+    UsageStatsResponse,
+)
 
 __all__ = [
     "UserRegister",
@@ -23,4 +31,10 @@ __all__ = [
     "ApiKeyUpdate",
     "ApiKeyResponse",
     "ApiKeyListResponse",
+    "UsageStatsCreate",
+    "UsageStatsResponse",
+    "StatsSummary",
+    "StatsByModel",
+    "StatsByDate",
+    "DashboardResponse",
 ]