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>

assets.py

@@ -1,5 +1,7 @@
+import copy
 import json
 import os
+from storage import load_json, save_json
 
 DATA_DIR = os.environ.get("DATA_DIR", os.path.join(os.path.dirname(__file__), "data"))
 ASSETS_FILE = os.path.join(DATA_DIR, "assets.json")
@@ -44,20 +46,19 @@ ASSET_CATEGORIES = ["Investments", "Real Estate", "Equipment", "Other Valuables"
 
 
 def load_assets():
-    if os.path.exists(ASSETS_FILE):
-        with open(ASSETS_FILE) as f:
-            data = json.load(f)
-            # migrate old format if needed
-            if "other_assets" not in data:
-                data["other_assets"] = []
-            return data
-    save_assets({"personal": DEFAULT_CARS, "flips": [], "other_assets": []})
-    return {"personal": DEFAULT_CARS, "flips": [], "other_assets": []}
+    data = load_json(ASSETS_FILE, None)
+    if data is None:
+        data = {"personal": copy.deepcopy(DEFAULT_CARS), "flips": [], "other_assets": []}
+        save_assets(data)
+        return data
+    # migrate old format if needed
+    if "other_assets" not in data:
+        data["other_assets"] = []
+    return data
 
 
 def save_assets(data):
-    with open(ASSETS_FILE, "w") as f:
-        json.dump(data, f, indent=2)
+    save_json(ASSETS_FILE, data)
 
 
 def car_display_name(car):