Add Docker support and docker-compose orchestration

Multi-stage Dockerfile for the Go API, plus a docker-compose.yml that
runs postgres + backend + both frontends together for local
full-stack dev (frontends built from the sibling repo directories).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,10 @@
+.git
+.idea
+.claude
+.env
+.env.*
+!.env.example
+uploads
+*.md
+/dating-app
+/dating-app-server