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>

.gitignore

@@ -21,6 +21,8 @@ backend/internal/graph/models_gen.go
 config.json
 settings.json
 *.db
+.env
+# .env.example is intentionally tracked — it contains no secrets
 
 # go.sum is intentionally committed once populated by go mod tidy