feat(A4): add MinIO service + storage abstraction (core/storage.py)

- Add MinIO service to docker-compose.yml (port 9000 API, 9001 console)
- Add minio-data volume for persistent object storage
- Create backend/app/core/storage.py: MinIOStorage + LocalStorage abstraction
  - MinIOStorage: boto3-based, auto-creates bucket, upload/download/exists/delete/presign
  - LocalStorage: fallback for dev (UPLOAD_DIR filesystem, backward compat)
  - get_storage() singleton: auto-selects based on MINIO_URL env var
- Add MINIO_URL/USER/PASSWORD/BUCKET env vars to all service definitions
- backend/pyproject.toml: docker>=6.1.0 → boto3>=1.34.0
- Add docker-compose.worker.yml: external render-worker for remote machines
- Fix .gitignore: 'core' rule was too broad, now only matches root /core dump
- Update .env.example: MinIO connection vars documented

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

docker-compose.yml

@@ -25,6 +25,23 @@ services:
       timeout: 5s
       retries: 5
 
+  minio:
+    image: minio/minio:latest
+    command: server /data --console-address ":9001"
+    environment:
+      MINIO_ROOT_USER: ${MINIO_USER:-minioadmin}
+      MINIO_ROOT_PASSWORD: ${MINIO_PASSWORD:-minioadmin}
+    ports:
+      - "9000:9000"
+      - "9001:9001"
+    volumes:
+      - minio-data:/data
+    healthcheck:
+      test: ["CMD", "curl", "-f", "http://localhost:9000/minio/health/live"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+
   backend:
     build:
       context: ./backend
@@ -46,6 +63,10 @@ services:
       - AZURE_OPENAI_API_VERSION=${AZURE_OPENAI_API_VERSION:-2024-02-01}
       - UPLOAD_DIR=/app/uploads
       - MAX_UPLOAD_SIZE_MB=${MAX_UPLOAD_SIZE_MB:-500}
+      - MINIO_URL=${MINIO_URL:-http://minio:9000}
+      - MINIO_USER=${MINIO_USER:-minioadmin}
+      - MINIO_PASSWORD=${MINIO_PASSWORD:-minioadmin}
+      - MINIO_BUCKET=${MINIO_BUCKET:-uploads}
     volumes:
       - ./backend:/app
       - uploads:/app/uploads
@@ -56,6 +77,8 @@ services:
         condition: service_healthy
       redis:
         condition: service_healthy
+      minio:
+        condition: service_healthy
 
   worker:
     build:
@@ -75,6 +98,10 @@ services:
       - AZURE_OPENAI_DEPLOYMENT=${AZURE_OPENAI_DEPLOYMENT:-gpt-4o}
       - AZURE_OPENAI_API_VERSION=${AZURE_OPENAI_API_VERSION:-2024-02-01}
       - UPLOAD_DIR=/app/uploads
+      - MINIO_URL=${MINIO_URL:-http://minio:9000}
+      - MINIO_USER=${MINIO_USER:-minioadmin}
+      - MINIO_PASSWORD=${MINIO_PASSWORD:-minioadmin}
+      - MINIO_BUCKET=${MINIO_BUCKET:-uploads}
       - CELERY_WORKER_CONCURRENCY=${CELERY_WORKER_CONCURRENCY:-8}
     volumes:
       - ./backend:/app
@@ -102,6 +129,10 @@ services:
       - UPLOAD_DIR=/app/uploads
       - BLENDER_BIN=/opt/blender/blender
       - RENDER_SCRIPTS_DIR=/render-scripts
+      - MINIO_URL=${MINIO_URL:-http://minio:9000}
+      - MINIO_USER=${MINIO_USER:-minioadmin}
+      - MINIO_PASSWORD=${MINIO_PASSWORD:-minioadmin}
+      - MINIO_BUCKET=${MINIO_BUCKET:-uploads}
     volumes:
       - ./backend:/app
       - uploads:/app/uploads
@@ -163,3 +194,4 @@ services:
 volumes:
   pgdata:
   uploads:
+  minio-data: