Align backend authentication and API response contracts

Replace cookie authentication with revocable Bearer sessions, optional one-use PNG challenges, bounded login limits, and short-lived market WebSocket tickets. Keep captcha disabled by default.

Standardize ordinary JSON responses while preserving HTTP statuses, raw OpenAPI, redirects, SSE, and WebSocket semantics. Revoke long connections with their owning session.

Expose official calendar data and collector status to the frontend and research tools, support bilingual news topics, and keep summaries and forward-only labels explicit. Verify the staged snapshot with all 305 backend tests.

backend/src/alphapulse/core/config.py

@@ -12,6 +12,7 @@ class Settings(BaseSettings):
     jwt_secret: str
     admin_username: str
     admin_password_hash: str = ""
+    login_captcha_enabled: bool = False
     jwt_expire_days: int = 7
     # Fernet 密钥，加密存库的第三方 API key
     secret_encryption_key: str