fix(usage): deduplicate proxy and session log usage records

Extract message_id from Claude API responses (msg_xxx) and use it to
generate a shared request_id format (session:{msg_xxx}) between the
proxy logger and session log sync. When session sync encounters the
same request_id via INSERT OR IGNORE, it skips the duplicate.

- Add message_id field to TokenUsage, extracted from Claude responses
- Add TokenUsage::dedup_request_id() to generate shared request IDs
- Define SESSION_REQUEST_ID_PREFIX constant to eliminate magic strings
- Change proxy logger to INSERT OR REPLACE for richer-data-wins semantics

src-tauri/src/services/session_usage_codex.rs

@@ -474,6 +474,7 @@ fn insert_codex_session_entry(
         cache_read_tokens: delta.cached_input,
         cache_creation_tokens: 0,
         model: Some(model.to_string()),
+        message_id: None,
     };
 
     let pricing = find_codex_pricing(&conn, model);