feat: migrate to Pydantic V2 and implement rate limiting middleware

- Migrate settings.py to Pydantic V2 (SettingsConfigDict, validation_alias)
- Update config models to use @field_validator with @classmethod
- Replace deprecated datetime.utcnow() with datetime.now(timezone.utc)
- Migrate FastAPI app from @app.on_event to lifespan context manager
- Implement comprehensive rate limiting middleware with:
  * Endpoint-specific rate limits (login: 5/min, register: 3/min)
  * IP-based and user-based tracking
  * Authenticated user multiplier (2x limits)
  * Bypass paths for health, docs, static, websocket endpoints
  * Rate limit headers in responses
- Add 13 comprehensive tests for rate limiting (all passing)
- Update instructions.md to mark completed tasks
- Fix asyncio.create_task usage in anime_service.py

All 714 tests passing. No deprecation warnings.

src/server/utils/error_tracking.py

@@ -6,7 +6,7 @@ for comprehensive error monitoring and debugging.
 """
 import logging
 import uuid
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import Any, Dict, Optional
 
 logger = logging.getLogger(__name__)
@@ -52,7 +52,7 @@ class ErrorTracker:
             Unique error tracking ID
         """
         error_id = str(uuid.uuid4())
-        timestamp = datetime.utcnow().isoformat()
+        timestamp = datetime.now(timezone.utc).isoformat()
 
         error_entry = {
             "id": error_id,
@@ -187,7 +187,7 @@ class RequestContextManager:
             "request_path": request_path,
             "request_method": request_method,
             "user_id": user_id,
-            "timestamp": datetime.utcnow().isoformat(),
+            "timestamp": datetime.now(timezone.utc).isoformat(),
         }
         self.context_stack.append(context)