feat: DB-first architecture with lock-safe writes

- price_monitor: writes live prices to both JSON and mofin.db (holdings + live_prices + portfolio_summary)
- mofin_db: added execute_with_retry/commit_with_retry with exponential backoff on 'database is locked'
- mofin_db: increased timeout 5s->15s, added PRAGMA busy_timeout=15000
- price_monitor retry loop: fixed break-before-if-ok bug (was not retrying on write failure)
- DB connection: WAL mode + retry decorator for all write operations
- cash sync: preserves DB authoritative cash (JSON cash not pushed to DB)

This is the DB-first version. JSON writes remain for dashboard compatibility.
Next step: remove JSON writes entirely for full DB-only architecture.

scripts/check_todos_schema.py

@@ -0,0 +1,8 @@
+import sqlite3
+
+for label, path in [("project", '/home/hmo/projects/MoFin/data/mofin.db'), ("real", '/home/hmo/web-dashboard/data/mofin.db')]:
+    db = sqlite3.connect(path)
+    sql = db.execute("SELECT sql FROM sqlite_master WHERE name='todos'").fetchone()
+    print(f"=== {label}: {path} ===")
+    print(sql[0] if sql else "NOT FOUND")
+    db.close()