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/models/config.py

@@ -1,6 +1,6 @@
 from typing import Dict, List, Optional
 
-from pydantic import BaseModel, Field, ValidationError, validator
+from pydantic import BaseModel, Field, ValidationError, field_validator
 
 
 class SchedulerConfig(BaseModel):
@@ -44,7 +44,8 @@ class LoggingConfig(BaseModel):
         default=3, ge=0, description="Number of rotated log files to keep"
     )
 
-    @validator("level")
+    @field_validator("level")
+    @classmethod
     def validate_level(cls, v: str) -> str:
         allowed = {"DEBUG", "INFO", "WARNING", "ERROR", "CRITICAL"}
         lvl = (v or "").upper()