Store SQLite DB on remote host via SCP for persistence

Instead of syncing JSON files back, the SQLite DB itself is now
the persistent store on the remote HAProxy host:
- Startup: download remote DB via SCP (skip migration if exists)
- After writes: upload local DB via SCP (WAL checkpoint first)
- JSON sync removed (sync_servers_json, sync_certs_json deleted)

New functions:
- ssh_ops: remote_download_file(), remote_upload_file() via SCP
- db: sync_db_to_remote(), _try_download_remote_db()

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/conftest.py

@@ -296,6 +296,7 @@ def patch_config_paths(temp_config_dir):
                 SERVERS_FILE=temp_config_dir["servers_file"],
                 CERTS_FILE=temp_config_dir["certs_file"],
                 DB_FILE=temp_config_dir["db_file"],
+                REMOTE_DB_FILE=temp_config_dir["db_file"],
             ):
                 # Patch health module which imports MAP_FILE and DB_FILE
                 with patch.multiple(