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>

debts.py

@@ -2,6 +2,7 @@ import json
 import os
 from datetime import date
 import math
+from storage import load_json, save_json
 
 DATA_DIR = os.environ.get("DATA_DIR", os.path.join(os.path.dirname(__file__), "data"))
 DEBTS_FILE = os.path.join(DATA_DIR, "debts.json")
@@ -116,27 +117,19 @@ OWNER_COLORS = {
 
 
 def load_debts():
-    if os.path.exists(DEBTS_FILE):
-        with open(DEBTS_FILE) as f:
-            return json.load(f)
-    return []
+    return load_json(DEBTS_FILE, [])
 
 
 def save_debts(debts):
-    with open(DEBTS_FILE, "w") as f:
-        json.dump(debts, f, indent=2)
+    save_json(DEBTS_FILE, debts)
 
 
 def load_debt_log():
-    if os.path.exists(DEBT_LOG_FILE):
-        with open(DEBT_LOG_FILE) as f:
-            return json.load(f)
-    return {}
+    return load_json(DEBT_LOG_FILE, {})
 
 
 def save_debt_log(log):
-    with open(DEBT_LOG_FILE, "w") as f:
-        json.dump(log, f, indent=2)
+    save_json(DEBT_LOG_FILE, log)
 
 
 def month_key(year, month):