feat: make docker-compose configurable via .env

Add environment variable support for HOST_PORT, DATA_VOLUME_PATH, and
FFMPEG_TIMEOUT with sensible defaults. Include .env.example template.

.env.example

@@ -0,0 +1,8 @@
+# Port exposed on host
+HOST_PORT=8000
+
+# Path to media files directory (mounted to /data in container)
+DATA_VOLUME_PATH=./data
+
+# Maximum job duration in seconds
+FFMPEG_TIMEOUT=3600