feat: persistent monitor_enabled flag + auto-start keepalive on boot

Makes live monitoring (and therefore alerting) genuinely 24/7 and
restart-surviving, instead of runtime-only keepalive.

- NL43Config.monitor_enabled (default True) + migrate_add_monitor_enabled.py.
- On startup, auto-start keepalive monitors for every monitor_enabled +
  tcp_enabled unit — so feeds/alerts resume after a restart with no manual step.
- /monitor/start and /monitor/stop now PERSIST monitor_enabled (start=True,
  stop=False) in addition to applying keepalive at runtime, so the toggle
  sticks. Roster output includes monitor_enabled for the admin UI to read.

On by default: configure a unit -> it's monitored 24/7 unless toggled off.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

migrate_add_monitor_enabled.py

@@ -0,0 +1,48 @@
+#!/usr/bin/env python3
+"""
+Migration: add monitor_enabled column to nl43_config.
+
+Controls whether the live fan-out DOD monitor is kept alive 24/7 for a unit
+(which is what makes alerting continuous). Defaults to enabled. Run once per DB.
+"""
+
+import sqlite3
+import sys
+from pathlib import Path
+
+DB_PATH = Path(__file__).parent / "data" / "slmm.db"
+
+
+def migrate():
+    if not DB_PATH.exists():
+        print(f"Database not found at {DB_PATH}")
+        print("No migration needed - database will be created with new schema")
+        return
+
+    conn = sqlite3.connect(DB_PATH)
+    cursor = conn.cursor()
+    try:
+        cursor.execute("PRAGMA table_info(nl43_config)")
+        columns = [row[1] for row in cursor.fetchall()]
+
+        if "monitor_enabled" in columns:
+            print("✓ monitor_enabled column already exists, no migration needed")
+            return
+
+        print("Adding monitor_enabled column (default enabled)...")
+        # SQLite stores booleans as 0/1; default 1 = enabled.
+        cursor.execute("ALTER TABLE nl43_config ADD COLUMN monitor_enabled BOOLEAN DEFAULT 1")
+        conn.commit()
+        print("✓ Added monitor_enabled column")
+        print("\n✓ Migration completed successfully!")
+
+    except Exception as e:
+        conn.rollback()
+        print(f"✗ Migration failed: {e}", file=sys.stderr)
+        sys.exit(1)
+    finally:
+        conn.close()
+
+
+if __name__ == "__main__":
+    migrate()