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/memory/mod.rs

@@ -115,6 +115,7 @@ pub fn create_memory(
             config.vector_weight as f32,
             config.keyword_weight as f32,
             config.embedding_cache_size,
+            config.sqlite_open_timeout_secs,
         )?;
         Ok(mem)
     }