fix: support concurrent MCP clients during server shutdown

[dapi]

Summary

• Verify that independent MCP sessions can make overlapping tool calls through one shared Telegram client and archive service.
• Let in-flight MCP POST tool calls finish before closing sessions, HTTP connections, the sync service, and the Telegram client.
• Document the supported single-server, multiple-client model.

Verification

• TGCLI_TEST_PORT="$(port-selector --name mcp-test)" npm test — 311 tests passed on current main.
• HTTP integration test: two MCP clients ran overlapping Telegram calls; archive read and sync-job scheduling completed while another call was pending; closing one session left the other usable.
• Queue concurrency test: overlapping worker starts processed one job once.
• Live npm start reached Telegram dialog initialization, then Telegram returned FLOOD_WAIT_23. Readiness with the personal session remains unverified. No chat content or credentials were included in test output.

Closes #33.