feat(schemas): T23 add Pydantic API key schemas

- Add ApiKeyCreate schema with OpenRouter key format validation
- Add ApiKeyUpdate schema for partial updates
- Add ApiKeyResponse schema (excludes key value for security)
- Add ApiKeyListResponse schema for pagination
- Export schemas from __init__.py
- 100% coverage on new module (23 tests)

Refs: T23

src/openrouter_monitor/schemas/__init__.py

@@ -1,4 +1,10 @@
 """Schemas package for OpenRouter Monitor."""
+from openrouter_monitor.schemas.api_key import (
+    ApiKeyCreate,
+    ApiKeyListResponse,
+    ApiKeyResponse,
+    ApiKeyUpdate,
+)
 from openrouter_monitor.schemas.auth import (
     TokenData,
     TokenResponse,
@@ -13,4 +19,8 @@ __all__ = [
     "UserResponse",
     "TokenResponse",
     "TokenData",
+    "ApiKeyCreate",
+    "ApiKeyUpdate",
+    "ApiKeyResponse",
+    "ApiKeyListResponse",
 ]