feat(db-postgres): initial implementation — pgx pool with lifecycle and UnitOfWork (v1.0.0)

Introduces code.nochebuena.dev/einherjar/db-postgres — the PostgreSQL database
starter for the Einherjar framework. Absorbs the postgres package from micro-lib,
replacing fmt.Errorf wrapping with core/xerrors and migrating from pgx v4 to pgx v5.

Interfaces (CT-6: one TypeSpec per file):
- Executor — Exec, Query, QueryRow (pgx-native types)
- Tx — Executor + Commit(ctx), Rollback(ctx)
- Provider — GetExecutor, Begin, BeginTx, Ping, HandleError
- Component — lifecycle.Component + observability.Checkable + Provider + Stats()
- UnitOfWork — Do(ctx, fn)

Implementation:
- New(logger, cfg) Component — pool not created until OnInit
- OnInit: pgxpool.NewWithConfig with duration parsing; 30s timeout
- OnStart: PING with 5s timeout; logs "postgres: connected"
- OnStop: pool.Close(); logs "postgres: closing pool"
- GetExecutor: returns active Tx from context (ctxTxKey) or pool; nil-safe
- Begin/BeginTx: wraps pgx.TxOptions; wrapped in xerrors on error
- HealthCheck: delegates to Ping; Priority LevelCritical
- Stats() *pgxpool.Stat — zero value when pool uninitialized
- NewUnitOfWork(logger, provider) UnitOfWork — Begin+inject+commit/rollback
- HandleError: UniqueViolation→ErrAlreadyExists, ForeignKey/Check→ErrInvalidInput,
  pgx.ErrNoRows→ErrNotFound, all others→ErrInternal

Config (EINHERJAR_PG_* env vars):
  Host, Port(5432), User, Password, Name, SSLMode(disable), Timezone(UTC),
  MaxConns(5), MinConns(2), MaxConnLifetime(1h), MaxConnIdleTime(30m),
  HealthCheckPeriod(1m)

- Component interface embeds observability.Identifiable; identifiable.go implements
  ModulePath and ModuleVersion via runtime/debug.ReadBuildInfo() — prints in launcher banner

errors.go

@@ -0,0 +1,45 @@
+package postgres
+
+import (
+	"errors"
+
+	"github.com/jackc/pgerrcode"
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgconn"
+
+	"code.nochebuena.dev/einherjar/core/xerrors"
+)
+
+// HandleError maps pgx and PostgreSQL errors to typed [xerrors] values.
+// Returns nil when err is nil. Also available as [Provider.HandleError].
+//
+// Mapped codes:
+//   - UniqueViolation     → ErrAlreadyExists
+//   - ForeignKeyViolation → ErrInvalidInput
+//   - CheckViolation      → ErrInvalidInput
+//   - pgx.ErrNoRows       → ErrNotFound
+//   - all others          → ErrInternal
+func HandleError(err error) error {
+	if err == nil {
+		return nil
+	}
+	var pgErr *pgconn.PgError
+	if errors.As(err, &pgErr) {
+		switch pgErr.Code {
+		case pgerrcode.UniqueViolation:
+			return xerrors.New(xerrors.ErrAlreadyExists, "record already exists").
+				WithContext("constraint", pgErr.ConstraintName).WithError(err)
+		case pgerrcode.ForeignKeyViolation:
+			return xerrors.New(xerrors.ErrInvalidInput, "data integrity violation").
+				WithContext("table", pgErr.TableName).WithError(err)
+		case pgerrcode.CheckViolation:
+			return xerrors.New(xerrors.ErrInvalidInput, "data constraint violation").
+				WithContext("table", pgErr.TableName).
+				WithContext("column", pgErr.ColumnName).WithError(err)
+		}
+	}
+	if errors.Is(err, pgx.ErrNoRows) {
+		return xerrors.New(xerrors.ErrNotFound, "record not found").WithError(err)
+	}
+	return xerrors.New(xerrors.ErrInternal, "unexpected database error").WithError(err)
+}