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

executor.go

@@ -0,0 +1,20 @@
+package postgres
+
+import (
+	"context"
+
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgconn"
+)
+
+// Executor is the shared query interface for both the connection pool and
+// an active transaction. Repository code accepts Executor so it works
+// identically inside and outside a [UnitOfWork].
+type Executor interface {
+	// Exec executes a query that returns no rows.
+	Exec(ctx context.Context, sql string, args ...any) (pgconn.CommandTag, error)
+	// Query executes a query that returns rows.
+	Query(ctx context.Context, sql string, args ...any) (pgx.Rows, error)
+	// QueryRow executes a query that returns at most one row.
+	QueryRow(ctx context.Context, sql string, args ...any) pgx.Row
+}