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/websocket.py

@@ -253,7 +253,7 @@ class ErrorNotificationMessage(BaseModel):
         description="Message type",
     )
     timestamp: str = Field(
-        default_factory=lambda: datetime.utcnow().isoformat(),
+        default_factory=lambda: datetime.now(timezone.utc).isoformat(),
         description="ISO 8601 timestamp",
     )
     data: Dict[str, Any] = Field(
@@ -274,7 +274,7 @@ class ProgressUpdateMessage(BaseModel):
         ..., description="Type of progress message"
     )
     timestamp: str = Field(
-        default_factory=lambda: datetime.utcnow().isoformat(),
+        default_factory=lambda: datetime.now(timezone.utc).isoformat(),
         description="ISO 8601 timestamp",
     )
     data: Dict[str, Any] = Field(