fix: docker build context and validation (#324) (#325)

Updates Dockerfile and Fly config to build from the project root context, allowing access to root-level files like README.md. Adds a CI workflow to validate the Docker build on PRs.

.github/workflows/deploy-prod.yml

···
           if [ "${{ steps.changes.outputs.migrations }}" == "true" ]; then
             echo "🔄 migrations detected - will run via release_command before deployment"
           fi
-          flyctl deploy --config backend/fly.toml --remote-only -a relay-api
+          flyctl deploy --config backend/fly.toml --remote-only -a relay-api .
         env:
           FLY_API_TOKEN: ${{ secrets.FLY_API_TOKEN_PROD }}

.github/workflows/deploy-staging.yml

···
           if [ "${{ steps.changes.outputs.migrations }}" == "true" ]; then
             echo "🔄 migrations detected - will run via release_command before deployment"
           fi
-          flyctl deploy --config backend/fly.staging.toml --remote-only -a relay-api-staging
+          flyctl deploy --config backend/fly.staging.toml --remote-only -a relay-api-staging .
         env:
           FLY_API_TOKEN: ${{ secrets.FLY_API_TOKEN_STAGING }}

README.md

···
 using [just](https://github.com/casey/just):
 
 ```bash
-# install dependencies
-cd backend && uv sync
+# install dependencies (uv handles backend venv automatically)
+uv sync # For root-level deps, if any, and initializes uv
 just frontend install
 
 # run backend (hot reloads at http://localhost:8001)
-just run-backend
+just backend run
 
 # run frontend (hot reloads at http://localhost:5173)
 just frontend dev

backend/fly.staging.toml

···
 primary_region = 'iad'
 
 [build]
-  dockerfile = "backend/Dockerfile"
-  ignore_file = "backend/.dockerignore"
+  dockerfile = "Dockerfile"
+  ignore_file = ".dockerignore"
 
 [deploy]
   release_command = 'uv run alembic upgrade head'

backend/fly.toml

···
 primary_region = 'iad'
 
 [build]
-  dockerfile = "backend/Dockerfile"
-  ignore_file = "backend/.dockerignore"
+  dockerfile = "Dockerfile"
+  ignore_file = ".dockerignore"
 
 [deploy]
   release_command = "uv run alembic upgrade head"

backend/justfile

···
 # backend/justfile
 set shell := ["bash", "-eu", "-o", "pipefail", "-c"]
-default := "run-backend"
+default := "run"
 
 # run backend server (hot reloads)
-run-backend:
+run:
     uv run uvicorn backend.main:app --reload --host 0.0.0.0 --port ${PORT:-8001}
 
 # run tests with docker-compose