feat(docker): bundle PostgreSQL into the unified container

Transition from a multi-service architecture to an all-in-one container
by bundling PostgreSQL directly within the Docker image. This simplifies
deployment, especially for environments like CasaOS, by removing the
need for an external database service.

- Update Dockerfile to install and configure PostgreSQL
- Implement database initialization logic in docker-entrypoint.sh
- Update .env.example to reflect auto-generation of credentials
- Simplify docker-compose.yml to a single service
- Update README.md with new deployment instructions and architecture details

.env.example

@@ -1,13 +1,14 @@
-# Trackeep Configuration for Casa OS
-# Only required variables - everything else is auto-configured
+# Trackeep All-in-One Configuration
+# PostgreSQL is bundled inside the container — no external database needed.
+# Everything below is optional; the container auto-generates sensible defaults.
 
-# Host port for the application (default: 8080)
+# Host port mapping (default: 8080)
 HOST_PORT=8080
 
-# Database Configuration
-DB_PASSWORD=your_secure_password_here
-DB_USER=trackeep
-DB_NAME=trackeep
+# Database credentials (auto-generated if left empty)
+# DB_PASSWORD=your_secure_password_here
+# DB_USER=trackeep
+# DB_NAME=trackeep
 
-# JWT Secret (generate with: openssl rand -hex 32)
-JWT_SECRET=your_jwt_secret_here_64_hex_characters_long_exactly
+# JWT Secret (auto-generated and persisted in /data if left empty)
+# JWT_SECRET=your_jwt_secret_here_64_hex_characters_long_exactly