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

config.go

@@ -0,0 +1,53 @@
+package postgres
+
+import (
+	"fmt"
+	"net/url"
+)
+
+// Config holds PostgreSQL connection settings. Required fields must be supplied
+// by the caller; optional fields have production-safe defaults via [DefaultConfig].
+type Config struct {
+	Host              string `env:"EINHERJAR_PG_HOST,required"`
+	Port              int    `env:"EINHERJAR_PG_PORT"                envDefault:"5432"`
+	User              string `env:"EINHERJAR_PG_USER,required"`
+	Password          string `env:"EINHERJAR_PG_PASSWORD,required"`
+	Name              string `env:"EINHERJAR_PG_NAME,required"`
+	SSLMode           string `env:"EINHERJAR_PG_SSL_MODE"            envDefault:"disable"`
+	Timezone          string `env:"EINHERJAR_PG_TIMEZONE"            envDefault:"UTC"`
+	MaxConns          int    `env:"EINHERJAR_PG_MAX_CONNS"           envDefault:"5"`
+	MinConns          int    `env:"EINHERJAR_PG_MIN_CONNS"           envDefault:"2"`
+	MaxConnLifetime   string `env:"EINHERJAR_PG_MAX_CONN_LIFETIME"   envDefault:"1h"`
+	MaxConnIdleTime   string `env:"EINHERJAR_PG_MAX_CONN_IDLE_TIME"  envDefault:"30m"`
+	HealthCheckPeriod string `env:"EINHERJAR_PG_HEALTH_CHECK_PERIOD" envDefault:"1m"`
+}
+
+// DefaultConfig returns a Config with all optional fields set to production-safe
+// defaults. Callers must supply Host, Port, User, Password, and Name.
+func DefaultConfig() Config {
+	return Config{
+		Port:              5432,
+		SSLMode:           "disable",
+		Timezone:          "UTC",
+		MaxConns:          5,
+		MinConns:          2,
+		MaxConnLifetime:   "1h",
+		MaxConnIdleTime:   "30m",
+		HealthCheckPeriod: "1m",
+	}
+}
+
+// DSN constructs a PostgreSQL connection string from the configuration.
+func (c Config) DSN() string {
+	u := &url.URL{
+		Scheme: "postgres",
+		User:   url.UserPassword(c.User, c.Password),
+		Host:   fmt.Sprintf("%s:%d", c.Host, c.Port),
+		Path:   "/" + c.Name,
+	}
+	q := u.Query()
+	q.Set("sslmode", c.SSLMode)
+	q.Set("timezone", c.Timezone)
+	u.RawQuery = q.Encode()
+	return u.String()
+}