feat(auth): add /v3/action/auth/authenticate_passcode endpoint

Moves passcode→JWT auth into the V3 action namespace. The new router
(api_v3_actions_auth.py) is a clean re-registration of the existing
working logic. The legacy /api/authenticate_passcode is kept but marked
deprecated via DeprecationParams while the frontend transitions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/routers/api.py

@@ -37,7 +37,7 @@ class PasscodeAuthRequest(BaseModel):
     site_id: str = Field(..., description="The random string ID of the site")
     passcode: str = Field(..., min_length=5, description="The passcode to verify")
 
-@router.post('/authenticate_passcode', response_model=Resp_Body_Base)
+@router.post('/authenticate_passcode', response_model=Resp_Body_Base, dependencies=[Depends(DeprecationParams)])
 async def authenticate_passcode(
         auth_req: PasscodeAuthRequest,
         response: Response = Response,