fix: Fix Docker compose startup issues (letta-ai#2056)

[sethanderson1]

Please describe the purpose of this pull request.
This PR fixes two Docker compose startup issues:

1. Removes duplicate nginx port listener configuration that causes nginx container to fail
2. Adds proper database healthcheck to ensure Letta server only starts after database is ready to accept connections

How to test

1. Pull the changes and run:

docker compose up

Expected outcomes:

• All three containers should start successfully, as shown:

[+] Running 4/3
 ✔ Network letta_default           Created                                                                                                              0.0s 
 ✔ Container letta-letta_db-1      Created                                                                                                              0.1s 
 ✔ Container letta-letta_nginx-1   Created                                                                                                              0.1s 
 ✔ Container letta-letta_server-1  Created

• No duplicate port listener error from nginx
• No database connection errors from Letta server
• System should be fully operational after startup completes

Have you tested this PR?
Yes, tested the latest commit. The system starts up cleanly as shown in the test output above, with all services starting and remaining running, with no connection errors.

Related issues or PRs
Fixes #2056

Is your PR over 500 lines of code?
No, this PR makes minimal targeted changes to nginx.conf and compose.yaml.

Additional context
These changes ensure reliable container startup sequence by:

• Cleaning up nginx configuration
• Using Docker's healthcheck feature to coordinate service startup
• Preventing race conditions in database connectivity

[sarahwooders]

Thanks for the contribution, looks great!