dev: switch to fully containerized Docker Compose workflow

- Replace tmux setup with a full Docker Compose development stack
- Update backend Dockerfile to use `cargo-chef` caching and `supervisord`
- Containerize frontend development server
- Update tests to run in isolated containers
- Remove legacy `papercrate.tmux`

backend/run-dev.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+set -e
+
+echo "Waiting for Postgres..."
+until pg_isready -h postgres -U papercrate; do
+  sleep 1
+done
+
+echo "Running migrations..."
+diesel migration run
+
+echo "Building binaries for first run..."
+cargo build --bin backend --bin worker --bin webdav
+
+echo "Starting supervisord..."
+exec supervisord -c supervisord.conf