Fix GET /mcp 405 log spam and resolve_account email matching

Add a custom GET /mcp health route so health checks and client polling
return 200 instead of flooding logs with 405. Also allow resolve_account
to match by email address (from_address, imap_username, smtp_username)
in addition to account ID.

Dockerfile

@@ -22,6 +22,6 @@ EXPOSE 3000
 ENV PORT=3000
 
 HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
-    CMD python -c "import httpx; r = httpx.get('http://localhost:3000/mcp', timeout=5); assert r.status_code == 405" || exit 1
+    CMD python -c "import httpx; r = httpx.get('http://localhost:3000/mcp', timeout=5); assert r.status_code == 200" || exit 1
 
 CMD ["python", "src/server.py"]