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

doc.go

@@ -0,0 +1,57 @@
+// Package postgres provides a pgx-backed PostgreSQL client with lifecycle management,
+// health checks, and unit-of-work transaction support for Einherjar applications.
+//
+// # Overview
+//
+// [New] returns a [Component] that manages a [pgxpool.Pool] connection pool,
+// satisfies the [lifecycle.Component] lifecycle hooks (OnInit, OnStart, OnStop),
+// and implements [observability.Checkable] with critical priority.
+//
+// [NewUnitOfWork] wraps multiple repository operations in a single transaction
+// via context injection — no transaction object is passed between functions.
+//
+// # Lifecycle Registration
+//
+//	db := postgres.New(logger, cfg)
+//	launcher.Register(db)
+//	health.Register(db)
+//
+// # Querying
+//
+// Repository code receives a [Provider] or [Executor] and calls [Provider.GetExecutor]
+// to obtain the active transaction (if inside a [UnitOfWork]) or the pool:
+//
+//	exec := db.GetExecutor(ctx)
+//	row := exec.QueryRow(ctx, "SELECT id FROM users WHERE email = $1", email)
+//	if err := row.Scan(&id); err != nil {
+//	    return db.HandleError(err)  // maps pgx.ErrNoRows → ErrNotFound, etc.
+//	}
+//
+// # Unit of Work
+//
+// [NewUnitOfWork] wraps operations in a single transaction. The transaction is
+// injected into the context; [Provider.GetExecutor] returns it automatically.
+//
+//	uow := postgres.NewUnitOfWork(logger, db)
+//	err := uow.Do(ctx, func(ctx context.Context) error {
+//	    exec := db.GetExecutor(ctx) // returns active Tx
+//	    _, err := exec.Exec(ctx, "INSERT INTO orders ...")
+//	    return err
+//	})
+//
+// # Error Handling
+//
+// [HandleError] translates pgx and PostgreSQL error codes into typed
+// [xerrors] values. Call it at every point where a pgx error is first observed:
+//
+//	if err := row.Scan(&out); err != nil {
+//	    return db.HandleError(err)
+//	}
+//
+// Mapped codes:
+//   - UniqueViolation     → ErrAlreadyExists
+//   - ForeignKeyViolation → ErrInvalidInput
+//   - CheckViolation      → ErrInvalidInput
+//   - pgx.ErrNoRows       → ErrNotFound
+//   - all others          → ErrInternal
+package postgres