T-4: add JPA, Liquibase, Testcontainers, and deployment docs

Set up development infrastructure for TDD: JPA + Liquibase for
database migrations, Testcontainers for integration tests against
real PostgreSQL, profile-based configuration (prod/local), and
README deployment documentation with docker-compose example.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -47,6 +47,7 @@ A privacy-focused, self-hostable web app for event announcements and RSVPs. An a
 
 - Java (latest LTS) + Maven wrapper (`./mvnw`, included)
 - Node.js (latest LTS) + npm
+- Docker (for running backend tests via Testcontainers)
 
 ### Project structure
 
@@ -68,6 +69,26 @@ cd backend && ./mvnw test
 cd frontend && npm run test:unit
 ```
 
+### Running the backend locally
+
+**Option A: Without external PostgreSQL (Testcontainers)**
+
+```bash
+cd backend && ./mvnw spring-boot:test-run
+```
+
+Starts the app with a Testcontainers-managed PostgreSQL that is created and destroyed automatically. Requires Docker.
+
+**Option B: With external PostgreSQL**
+
+```bash
+cd backend
+cp src/main/resources/application-local.properties.example \
+   src/main/resources/application-local.properties
+# Edit application-local.properties if your PostgreSQL uses different credentials
+./mvnw spring-boot:run -Dspring-boot.run.profiles=local
+```
+
 ### Building
 
 ```bash
@@ -129,6 +150,52 @@ ArchUnit enforces hexagonal boundaries: domain must not depend on adapters, appl
 |---------------------|------------------|-------------------|
 | Prettier            | `npm run format` | Formatting issues |
 
+## Deployment
+
+### Docker Compose
+
+The app ships as a single Docker image. It requires an external PostgreSQL database.
+
+```yaml
+services:
+  db:
+    image: postgres:17-alpine
+    environment:
+      POSTGRES_DB: fete
+      POSTGRES_USER: fete
+      POSTGRES_PASSWORD: changeme
+    volumes:
+      - fete-db:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U fete"]
+      interval: 5s
+      timeout: 3s
+      retries: 5
+
+  app:
+    image: git.bahamut.nitrix.one/nitrix/fete:latest
+    ports:
+      - "8080:8080"
+    environment:
+      DATABASE_URL: jdbc:postgresql://db:5432/fete
+      DATABASE_USERNAME: fete
+      DATABASE_PASSWORD: changeme
+    depends_on:
+      db:
+        condition: service_healthy
+
+volumes:
+  fete-db:
+```
+
+### Environment variables
+
+| Variable            | Required | Default | Description                       |
+|---------------------|----------|---------|-----------------------------------|
+| `DATABASE_URL`      | Yes      | —       | JDBC connection string            |
+| `DATABASE_USERNAME` | Yes      | —       | Database username                 |
+| `DATABASE_PASSWORD` | Yes      | —       | Database password                 |
+
 ## License
 
 GPL — see [LICENSE](LICENSE) for details.