fix: replace hardcoded DB credentials with env var interpolation in docker-compose.yml

All DATABASE_URL values and postgres service env vars now use
\${POSTGRES_USER}, \${POSTGRES_PASSWORD}, \${POSTGRES_DB} interpolation
instead of hardcoded ml_user/ml_password/candle_annotator values.
Also updated pg_isready healthcheck to use the same env vars.

Closes task 1.5.

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

docker-compose.yml

@@ -7,7 +7,7 @@ services:
       - ml-data:/app/ml-data
     environment:
       - NODE_ENV=production
-      - DATABASE_URL=postgresql://ml_user:ml_password@postgres:5432/candle_annotator
+      - DATABASE_URL=postgresql://${POSTGRES_USER}:${POSTGRES_PASSWORD}@postgres:5432/${POSTGRES_DB}
       - INFERENCE_API_URL=http://ml-service:8001
       - INFERENCE_API_TIMEOUT=30000
       - INFERENCE_BATCH_TIMEOUT=120000
@@ -34,7 +34,7 @@ services:
       - mlflow-data:/app/mlruns
     environment:
       - MLFLOW_TRACKING_URI=http://mlflow:5000
-      - DATABASE_URL=postgresql://ml_user:ml_password@postgres:5432/candle_annotator
+      - DATABASE_URL=postgresql://${POSTGRES_USER}:${POSTGRES_PASSWORD}@postgres:5432/${POSTGRES_DB}
       - PYTHONUNBUFFERED=1
     restart: unless-stopped
     depends_on:
@@ -70,12 +70,12 @@ services:
     volumes:
       - postgres-data:/var/lib/postgresql/data
     environment:
-      - POSTGRES_USER=ml_user
-      - POSTGRES_PASSWORD=ml_password
-      - POSTGRES_DB=candle_annotator
+      - POSTGRES_USER=${POSTGRES_USER}
+      - POSTGRES_PASSWORD=${POSTGRES_PASSWORD}
+      - POSTGRES_DB=${POSTGRES_DB}
     restart: unless-stopped
     healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U ml_user -d candle_annotator"]
+      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER} -d ${POSTGRES_DB}"]
       interval: 10s
       timeout: 5s
       retries: 5