docs: add production deployment example with volumes and .env

- docker/docker-compose.prod.yml: pulls image from 192.168.0.19:5000,
  mounts config/, wiki/ and db/ as separate volumes, health check, TZ support
- .env.example: fully commented template for all env vars
- .gitignore: ignore .env, always track .env.example
- README: deployment section with step-by-step guide, volume table, backup commands
- config.go: default db_path to /data/db/archivum.db (matches prod volume layout)

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

backend/internal/config/config.go

@@ -49,6 +49,12 @@ func Load(path string) (*Config, error) {
 		cfg.ListenAddr = ":4000"
 	}
 
+	// Default DB path to the dedicated db volume so it can be backed up
+	// independently of the wiki content.
+	if cfg.DBPath == "" {
+		cfg.DBPath = "/data/db/archivum.db"
+	}
+
 	return &cfg, nil
 }