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/services/anime_service.py

@@ -115,14 +115,18 @@ class AnimeService:
                     total = progress_data.get("total", 0)
                     message = progress_data.get("message", "Scanning...")
                     
-                    asyncio.create_task(
-                        self._progress_service.update_progress(
-                            progress_id=scan_id,
-                            current=current,
-                            total=total,
-                            message=message,
+                    # Schedule the coroutine without waiting for it
+                    # This is safe because we don't need the result
+                    loop = asyncio.get_event_loop()
+                    if loop.is_running():
+                        asyncio.ensure_future(
+                            self._progress_service.update_progress(
+                                progress_id=scan_id,
+                                current=current,
+                                total=total,
+                                message=message,
+                            )
                         )
-                    )
                 except Exception as e:
                     logger.error("Scan progress callback error", error=str(e))