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.

tests/unit/test_download_service.py

@@ -7,7 +7,7 @@ from __future__ import annotations
 
 import asyncio
 import json
-from datetime import datetime
+from datetime import datetime, timezone
 from pathlib import Path
 from unittest.mock import AsyncMock, MagicMock
 
@@ -84,7 +84,7 @@ class TestDownloadServiceInitialization:
                     "episode": {"season": 1, "episode": 1, "title": None},
                     "status": "pending",
                     "priority": "normal",
-                    "added_at": datetime.utcnow().isoformat(),
+                    "added_at": datetime.now(timezone.utc).isoformat(),
                     "started_at": None,
                     "completed_at": None,
                     "progress": None,
@@ -95,7 +95,7 @@ class TestDownloadServiceInitialization:
             ],
             "active": [],
             "failed": [],
-            "timestamp": datetime.utcnow().isoformat(),
+            "timestamp": datetime.now(timezone.utc).isoformat(),
         }
 
         with open(persistence_file, "w", encoding="utf-8") as f: