Harden JSON storage (atomic writes + crash-safe loads) + suggestions lightbulb

- storage.py: atomic save_json (temp+fsync+os.replace) and load_json that
  survives missing/corrupt files (moves corrupt aside). Closes #11/#12.
- route all data modules + app.py helpers through storage (closes #31 core);
  fixes mutable DEFAULT_FUNDS/DEFAULT_CARS return (#23).
- gunicorn --workers 1 --threads 4 to remove the write race (#13); strip
  Dockerfile template scaffolding (#40).
- base.html: 💡 suggestions link in the shared nav (shows on every page).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

mortgage.py

@@ -3,21 +3,18 @@ import os
 from datetime import date
 from dateutil.relativedelta import relativedelta
 import math
+from storage import load_json, save_json
 
 DATA_DIR = os.environ.get("DATA_DIR", os.path.join(os.path.dirname(__file__), "data"))
 MORTGAGE_FILE = os.path.join(DATA_DIR, "mortgage.json")
 
 
 def load_mortgage():
-    if os.path.exists(MORTGAGE_FILE):
-        with open(MORTGAGE_FILE) as f:
-            return json.load(f)
-    return {}
+    return load_json(MORTGAGE_FILE, {})
 
 
 def save_mortgage(data):
-    with open(MORTGAGE_FILE, "w") as f:
-        json.dump(data, f, indent=2)
+    save_json(MORTGAGE_FILE, data)
 
 
 def monthly_payment(principal, annual_rate, term_years):