fix: Add support for external PostgreSQL and Redis instances

Implements smart detection that allows users to provide external DATABASE_URL
or REDIS_URL. When external services are detected, internal instances are
automatically disabled to save resources. Maintains full backward compatibility
with existing setup

docker-compose.yml

@@ -53,6 +53,24 @@ services:
       # CONFIG_ENCRYPTION_KEY: "your-custom-encryption-key-here"
       # POSTGRES_PASSWORD: "your-custom-postgres-password-here"
 
+      # ========================================================================
+      # OPTIONAL: External PostgreSQL and Redis
+      # ========================================================================
+      # To use external PostgreSQL or Redis instances instead of the internal ones,
+      # uncomment and configure the appropriate URL(s):
+      #
+      # External PostgreSQL example:
+      # DATABASE_URL: "postgresql://username:password@postgres.example.com:5432/readmeabook"
+      #
+      # External Redis example:
+      # REDIS_URL: "redis://redis.example.com:6379"
+      # REDIS_URL: "redis://:password@redis.example.com:6379"  # With password
+      #
+      # Note: When using external services:
+      # - The internal PostgreSQL/Redis will NOT start (smart detection)
+      # - You do NOT need to mount ./pgdata or ./redis volumes
+      # - Ensure your external services are accessible from the container
+
       # ========================================================================
       # OPTIONAL: Rootless Podman Support
       # ========================================================================