feat(memory): optional SQLite connection open timeout

- Add memory.sqlite_open_timeout_secs config (None = wait indefinitely).
- When set, open the DB in a thread with recv_timeout; cap at 300s.
- Default remains None for backward compatibility.
- Document in README; add tests for timeout path and default.

src/onboard/wizard.rs

@@ -288,6 +288,7 @@ fn memory_config_defaults_for_backend(backend: &str) -> MemoryConfig {
         snapshot_enabled: false,
         snapshot_on_hygiene: false,
         auto_hydrate: true,
+        sqlite_open_timeout_secs: None,
     }
 }