refactor(constants): single source of truth for data dir (#3368)

* refactor(constants): single source of truth for data dir + merge core/src constants

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* docs(contributing): use named src.constants for data paths, drop core/constants references

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

routes/email_routes.py

@@ -32,6 +32,7 @@ from email.mime.multipart import MIMEMultipart
 
 from fastapi import APIRouter, Query, UploadFile, File, BackgroundTasks, HTTPException, Depends, Request
 from fastapi.responses import FileResponse
+from src.constants import DATA_DIR
 
 from src.llm_core import llm_call_async
 from src.upload_limits import read_upload_limited
@@ -2904,7 +2905,7 @@ def setup_email_routes():
         from pathlib import Path as _P
         import json as _json
         _slug = "".join(c if (c.isalnum() or c in "-_.@") else "_" for c in (owner or "default"))
-        path = _P(f"data/email_urgency_state_{_slug}.json")
+        path = _P(DATA_DIR) / f"email_urgency_state_{_slug}.json"
         if not path.exists():
             return {"total_unread": 0, "total_urgent": 0, "max_score": 0, "per_uid": {}}
         try: