docs(06-02): create Diátaxis documentation for Lab 05 Database & RDS

Documentation (6 files, 1500+ lines):
Tutorials (3):
- 01-deploy-rds-database.md: Deploy PostgreSQL in private network
- 02-data-persistence.md: Data persistence with named volumes
- 03-security-compliance.md: INF-01/02/03/04 compliance

How-to Guides (1):
- connect-to-postgresql.md: Connection methods

Reference (1):
- postgresql-commands.md: PostgreSQL command reference

Explanation (1):
- database-rds-parallels.md: Docker↔RDS parallels with architecture diagrams

Key concepts:
- PostgreSQL container → RDS Instance
- Private network → VPC Private Subnet
- Named volume → EBS volume
- Resource limits → DB instance class

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

labs/lab-05-database/how-to-guides/connect-to-postgresql.md

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+# How-to: Connettersi a PostgreSQL in Docker
+
+## Come connettersi al database
+
+### Da container nella stessa rete
+
+```bash
+docker exec lab05-app psql -h db -U lab05_user -d lab05_db
+```
+
+### Dall'host con port forwarding (non recommended)
+
+Se devi connetterti dall'host, aggiungi temporaneamente:
+
+```yaml
+# docker-compose.yml
+ports:
+  - "127.0.0.1:5432:5432"
+```
+
+Poi:
+
+```bash
+psql -h 127.0.0.1 -U lab05_user -d lab05_db
+```
+
+### Con pgAdmin o altri tool
+
+1. Configura connection string:
+```
+host=127.0.0.1 port=5432 user=lab05_user password=lab05_password dbname=lab05_db
+```
+
+2. Oppure usa PostgreSQL URI:
+```
+postgresql://lab05_user:lab05_password@127.0.0.1:5432/lab05_db
+```
+
+## Risoluzione problemi
+
+### Connection refused
+
+Il database è in rete privata. Devi connetterti da container nella stessa rete:
+
+```bash
+# Prima entra in un container
+docker exec -it lab05-app sh
+
+# Poi connettiti
+psql -h db -U lab05_user -d lab05_db
+```
+
+### Password authentication failed
+
+Verifica le credenziali in docker-compose.yml:
+
+```bash
+grep POSTGRES_PASSWORD docker-compose.yml
+```