Add standalone PostgreSQL install app

[CarlosZiegler]

Summary

Adds a standalone PostgreSQL app to the Install App catalog.

This gives users a simpler option when they only need Postgres, without installing the full Supabase stack.

Changes

• Add a postgres catalog template using postgres:16-alpine
• Generate the database password during install
• Persist data in a named Docker volume
• Expose the PostgreSQL TCP endpoint
• Show connection outputs, including public and internal DATABASE_URL variants
• Add PostgreSQL to the featured apps list
• Add a PostgreSQL logo mapping for the catalog card
• Regenerate the merged app catalog

Closes #402.

Verification

• bun packages/core/scripts/gen-catalog.ts
• bun --filter @repo/core test -- src/apps/catalog.test.ts
• bun --filter @repo/core lint
• git diff --check
• cd apps/dashboard && bun x tsc --noEmit

Note: the existing dashboard lint script currently fails with next lint under the repo's Next.js version before checking this diff. I used a dashboard typecheck instead for the touched UI files.