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/note_routes.py

@@ -11,6 +11,7 @@ from pydantic import BaseModel
 
 from core.database import SessionLocal, Note
 from src.auth_helpers import get_current_user
+from src.constants import DATA_DIR
 from sqlalchemy.orm.attributes import flag_modified
 
 logger = logging.getLogger(__name__)
@@ -170,7 +171,7 @@ async def dispatch_reminder(
             from datetime import datetime as _dt, timezone as _tz, timedelta as _td
             from pathlib import Path as _P
             _slug = "".join(c if (c.isalnum() or c in "-_.@") else "_" for c in (owner or "default"))
-            cache_path = _P(f"data/note_pings_{_slug}.json")
+            cache_path = _P(DATA_DIR) / f"note_pings_{_slug}.json"
             if cache_path.exists():
                 cache = _json.loads(cache_path.read_text(encoding="utf-8"))
             last = cache.get(cache_key)
@@ -523,7 +524,7 @@ async def dispatch_reminder(
             _STATE = cache_path
             if _STATE is None:
                 _slug = "".join(c if (c.isalnum() or c in "-_.@") else "_" for c in (owner or "default"))
-                _STATE = _P(f"data/note_pings_{_slug}.json")
+                _STATE = _P(DATA_DIR) / f"note_pings_{_slug}.json"
             _STATE.parent.mkdir(parents=True, exist_ok=True)
             try:
                 _cache = cache or (_json.loads(_STATE.read_text(encoding="utf-8")) if _STATE.exists() else {})