feat(postgres): initial stable release v0.9.0

pgx v5-native PostgreSQL client with launcher lifecycle, health check, unit-of-work via context injection, and structured error mapping.

What's included:
- Executor / Tx / Client / Component interfaces using pgx native types (pgconn.CommandTag, pgx.Rows, pgx.Row)
- New(logger, cfg) constructor; pgxpool initialised in OnInit
- Config struct with env-tag support for all pool tuning parameters
- UnitOfWork via context injection; GetExecutor(ctx) returns active Tx or pool
- HandleError mapping pgerrcode constants to xerrors codes (AlreadyExists, InvalidInput, NotFound, Internal)
- health.Checkable at LevelCritical; HealthCheck delegates to pgxpool.Ping

Tested-via: todo-api POC integration
Reviewed-against: docs/adr/

compliance_test.go

@@ -0,0 +1,9 @@
+package postgres_test
+
+import (
+	"code.nochebuena.dev/go/logz"
+	"code.nochebuena.dev/go/postgres"
+)
+
+// Compile-time check: New returns a valid Component.
+var _ postgres.Component = postgres.New(logz.New(logz.Options{}), postgres.Config{})