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_db_paths.py

@@ -0,0 +1,22 @@
+import sqlite3
+
+# Script connects to THIS db
+proj_db = '/home/hmo/projects/MoFin/data/mofin.db'
+# Real data lives in THIS db
+real_db = '/home/hmo/web-dashboard/data/mofin.db'
+
+for label, path in [("project", proj_db), ("real", real_db)]:
+    db = sqlite3.connect(path)
+    tables = [r[0] for r in db.execute("SELECT name FROM sqlite_master WHERE type='table'")]
+    print(f"{label} db ({path}): {len(tables)} tables")
+    for t in tables:
+        if t == 'todos':
+            sql = db.execute(f"SELECT sql FROM sqlite_master WHERE name='{t}'").fetchone()
+            print(f"  {t}: {sql[0][:100] if sql else 'no sql'}")
+        elif t in ('holdings', 'holding_strategies', 'watchlist_stocks', 'portfolio_summary'):
+            cnt = db.execute(f"SELECT COUNT(*) FROM {t}").fetchone()[0]
+            print(f"  {t}: {cnt} rows")
+        else:
+            cnt = db.execute(f"SELECT COUNT(*) FROM {t}").fetchone()[0]
+            print(f"  {t}: {cnt} rows")
+    db.close()