refactor: replace local _get_data_dir() with shared app.utils.get_data_dir()

Replace copy-pasted _get_data_dir() functions in catalog.py and upload.py
with import from app.utils.get_data_dir(). sync.py and data.py already use
the shared utility.

app/api/catalog.py

@@ -1,25 +1,20 @@
 """Catalog endpoints — table profiles, metrics."""
 
 import json
-import os
 import re
-from pathlib import Path
 
 from fastapi import APIRouter, Depends, HTTPException
 import duckdb
 import yaml
 
 from app.auth.dependencies import get_current_user, _get_db
+from app.utils import get_data_dir as _get_data_dir
 from src.repositories.profiles import ProfileRepository
 from src.rbac import can_access_table
 
 router = APIRouter(prefix="/api/catalog", tags=["catalog"])
 
 
-def _get_data_dir() -> Path:
-    return Path(os.environ.get("DATA_DIR", "./data"))
-
-
 @router.get("/profile/{table_name}")
 async def get_table_profile(
     table_name: str,

app/api/upload.py

@@ -1,23 +1,18 @@
 """Upload endpoints — sessions, artifacts, CLAUDE.local.md."""
 
-import os
 import uuid
 from datetime import datetime, timezone
-from pathlib import Path
 from pathlib import Path as _Path
 
 from fastapi import APIRouter, Depends, HTTPException, UploadFile, File
 from pydantic import BaseModel
 
 from app.auth.dependencies import get_current_user
+from app.utils import get_data_dir as _get_data_dir
 
 router = APIRouter(prefix="/api/upload", tags=["upload"])
 
 
-def _get_data_dir() -> Path:
-    return Path(os.environ.get("DATA_DIR", "./data"))
-
-
 @router.post("/sessions")
 async def upload_session(
     file: UploadFile = File(...),