fix: rm docker approach to dbmate migration creation

[samrose]

removes the Docker-based workflow for creating database migrations with dbmate. Specifically, it deletes:

1. migrations/Dockerfile.dbmate (23 lines removed)
   - Custom Dockerfile for running dbmate with PostgreSQL client
   - Supported both amd64 and arm64 architectures
   - Installed dbmate from GitHub releases and PostgreSQL client tools
2. migrations/docker-compose.yaml (46 lines removed)
   - Docker Compose setup with 3 services: db, pg_prove, and dbmate
   - Used supabase_postgres image for the database
   - Configured health checks and service dependencies
   - Set up pg_prove for running tests after migrations
3. migrations/README.md (17 lines removed)
   - Removed documentation section: "Adding a migration with docker-compose"
   - Deleted instructions for using docker-compose up and docker-compose run --rm dbmate commands

Rationale:

1. Image used for migration management contains vulnerabilities
2. We have a fully documented/supported way to create and manage migrations with nix. To create migrations going forward, we just run nix develop and then run dbmate directly, or nix develop -c dbmate ... [etc]