fix: ensure Postgres init runs so signup and login work

[Recongohan]

🔗 Linked issue

Resolves #169

Description

This PR fixes an issue where running docker compose up would start the application, but signup and login failed because the users table was never created.

Root cause
The Postgres init.sql file was not mounted into the Postgres init directory, so schema initialization never ran. The init.sql file also contained schema issues that caused initialization to fail when executed.

Changes
Mounted init.sql into the Postgres init directory in docker compose
Fixed init.sql so required tables are created successfully
Added a README troubleshooting note to help contributors reset local DB state

Testing
Started from a clean local database state
Verified tables using psql
Successfully signed up and logged in on localhost 4000

Checklist

• I've followed the Contributing guidelines
• I've titled my PR according to the Conventional Commits spec
• I've linked an open issue
• I've added tests that fail without this PR but pass with it
• I've linted and tested my code
• I've updated documentation if appropriate