Migrate from TinyDB to SQLite

- Replace TinyDB (JSON file) with sqlite3 for data persistence
- Add schema.py: table creation + data migration from db.json
- Rewrite database.py: all CRUD operations use sqlite3 directly
- All data retains original IDs via migration script
- Remove tinydb dependency from pyproject.toml

static/js/goals.js

@@ -60,7 +60,7 @@ function openModal(goal = null) {
     
     if (goal) {
         modalTitle.textContent = "Edit Goal";
-        goalId.value = goal.doc_id;
+        goalId.value = goal.id;
         goalTitle.value = goal.title;
     } else {
         modalTitle.textContent = "Create Goal";