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/proxy/usage/logger.rs

@@ -73,7 +73,7 @@ impl<'a> UsageLogger<'a> {
             });
 
         conn.execute(
-            "INSERT INTO proxy_request_logs (
+            "INSERT OR REPLACE INTO proxy_request_logs (
                 request_id, provider_id, app_type, model, request_model,
                 input_tokens, output_tokens, cache_read_tokens, cache_creation_tokens,
                 input_cost_usd, output_cost_usd, cache_read_cost_usd, cache_creation_cost_usd, total_cost_usd,
@@ -358,6 +358,7 @@ mod tests {
             cache_read_tokens: 0,
             cache_creation_tokens: 0,
             model: None,
+            message_id: None,
         };
 
         logger.log_with_calculation(