feat(pgdb): add DATABASE_URI environment variable support

Add support for PostgreSQL connection URIs via DATABASE_URI env var.
When set, it takes precedence over config files and PoolOptions are
ignored (pool settings can be specified in URI query string).

database/pgdb/CLAUDE.md

@@ -93,8 +93,34 @@ sslmode: prefer
 
 ## Environment Variables
 
+- `DATABASE_URI` - PostgreSQL connection URI (takes precedence over config files)
 - `DATABASE_CONFIG_FILE` - Override config file location
 
+### URI Format
+
+Standard PostgreSQL URI format:
+```
+postgresql://user:password@host:port/database?sslmode=require&pool_max_conns=10
+```
+
+Pool settings can be included in the URI query string:
+- `pool_max_conns`, `pool_min_conns`
+- `pool_max_conn_lifetime`, `pool_max_conn_idle_time`
+- `pool_health_check_period`
+
+When using `DATABASE_URI`, `PoolOptions` are ignored and all settings come from the URI.
+
+Example with CloudNativePG:
+```yaml
+# Mount the secret's 'uri' key as DATABASE_URI
+env:
+  - name: DATABASE_URI
+    valueFrom:
+      secretKeyRef:
+        name: mydb-app
+        key: uri
+```
+
 ## When to Use
 
 **Use `pgdb.OpenPool()`** (this package) when: