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/reference/postgresql-commands.md

@@ -0,0 +1,91 @@
+# Reference: PostgreSQL Commands per Lab 05
+
+## Comandi Utili
+
+### Connessione
+
+```bash
+# Connessione base
+docker exec lab05-db psql -U lab05_user -d lab05_db
+
+# Connessione con host specificato
+docker exec lab05-app psql -h db -U lab05_user -d lab05_db
+
+# Esegui comando singolo
+docker exec lab05-db psql -U lab05_user -d lab05_db -c "SELECT version();"
+```
+
+### Operazioni Database
+
+```sql
+-- Crea database
+CREATE DATABASE test_db;
+
+-- Lista database
+\l
+
+-- Cambia database
+\c test_db
+
+-- Droppia database
+DROP DATABASE test_db;
+```
+
+### Operazioni Tabelle
+
+```sql
+-- Crea tabella
+CREATE TABLE test_table (
+    id SERIAL PRIMARY KEY,
+    name TEXT,
+    value INTEGER
+);
+
+-- Lista tabelle
+\dt
+
+-- Descrivi tabella
+\d test_table
+
+-- Droppia tabella
+DROP TABLE test_table;
+```
+
+### DML
+
+```sql
+-- Insert
+INSERT INTO test_table (name, value) VALUES ('test', 42);
+
+-- Select
+SELECT * FROM test_table;
+
+-- Update
+UPDATE test_table SET value = 100 WHERE name = 'test';
+
+-- Delete
+DELETE FROM test_table WHERE name = 'test';
+```
+
+### Utility
+
+```bash
+# Verifica PostgreSQL pronto
+docker exec lab05-db pg_isready -U lab05_user
+
+# Backup database
+docker exec lab05-db pg_dump -U lab05_user lab05_db > backup.sql
+
+# Ripristina database
+cat backup.sql | docker exec -i lab05-db psql -U lab05_user lab05_db
+```
+
+## Variabili Ambiente PostgreSQL
+
+| Variabile | Descrizione | Default |
+|-----------|-------------|---------|
+| POSTGRES_DB | Nome database | postgres |
+| POSTGRES_USER | Utente creato | - |
+| POSTGRES_PASSWORD | Password utente | - |
+| POSTGRES_INITDB_ARGS | Argomenti initdb | - |
+| PGDATA | Data directory | /var/lib/postgresql/data |