Stage 1: backend foundation (Postgres, sessions, accounts, OTel)

- internal/postgres: pgx-over-database/sql pool (otelsql), embedded goose
  migrations into schema 'backend', committed go-jet code + cmd/jetgen tool.
- internal/account: durable accounts + unified telegram/email identities
  (UUIDv7 keys), find-or-create provisioning with unique-conflict handling.
- internal/session: opaque 256-bit tokens stored as a SHA-256 hash, revoke-only
  (no TTL); write-through cache gating /readyz; store + service.
- internal/telemetry: OTel tracer/meter providers (none/stdout) + request-timing
  middleware; internal/config gains Postgres + OTel env loading.
- internal/server: /api/v1 {public,user,internal,admin} skeleton + X-User-ID
  middleware; /readyz checks DB ping + cache; main wires
  telemetry -> db+migrate -> warm cache -> server.
- Tests: unit + integration (build tag 'integration', testcontainers
  postgres:17) for migrations, accounts, sessions, readyz; new integration.yaml.
- Docs: ARCHITECTURE, TESTING, PLAN refinements, root + backend READMEs.

Session/account REST handlers deferred to Stage 6 (gateway); OTLP + dashboards
to Stage 11.

backend/internal/config/config.go

@@ -5,6 +5,11 @@ package config
 import (
 	"fmt"
 	"os"
+	"strconv"
+	"time"
+
+	"scrabble/backend/internal/postgres"
+	"scrabble/backend/internal/telemetry"
 )
 
 // Config holds the backend's runtime configuration.
@@ -13,6 +18,10 @@ type Config struct {
 	HTTPAddr string
 	// LogLevel is the zap log level: "debug", "info", "warn" or "error".
 	LogLevel string
+	// Postgres configures the primary database pool.
+	Postgres postgres.Config
+	// Telemetry configures the OpenTelemetry providers.
+	Telemetry telemetry.Config
 }
 
 // Defaults applied when the corresponding environment variable is unset.
@@ -21,12 +30,35 @@ const (
 	defaultLogLevel = "info"
 )
 
-// Load reads the configuration from the environment, applies defaults for
-// unset variables, and validates the result.
+// Load reads the configuration from the environment, applies defaults for unset
+// variables, and validates the result.
 func Load() (Config, error) {
+	pg := postgres.DefaultConfig()
+	pg.DSN = os.Getenv("BACKEND_POSTGRES_DSN")
+	var err error
+	if pg.MaxOpenConns, err = envInt("BACKEND_POSTGRES_MAX_OPEN_CONNS", pg.MaxOpenConns); err != nil {
+		return Config{}, err
+	}
+	if pg.MaxIdleConns, err = envInt("BACKEND_POSTGRES_MAX_IDLE_CONNS", pg.MaxIdleConns); err != nil {
+		return Config{}, err
+	}
+	if pg.ConnMaxLifetime, err = envDuration("BACKEND_POSTGRES_CONN_MAX_LIFETIME", pg.ConnMaxLifetime); err != nil {
+		return Config{}, err
+	}
+	if pg.OperationTimeout, err = envDuration("BACKEND_POSTGRES_OPERATION_TIMEOUT", pg.OperationTimeout); err != nil {
+		return Config{}, err
+	}
+
+	tel := telemetry.DefaultConfig()
+	tel.ServiceName = envOr("BACKEND_SERVICE_NAME", tel.ServiceName)
+	tel.TracesExporter = envOr("BACKEND_OTEL_TRACES_EXPORTER", tel.TracesExporter)
+	tel.MetricsExporter = envOr("BACKEND_OTEL_METRICS_EXPORTER", tel.MetricsExporter)
+
 	c := Config{
-		HTTPAddr: envOr("BACKEND_HTTP_ADDR", defaultHTTPAddr),
-		LogLevel: envOr("BACKEND_LOG_LEVEL", defaultLogLevel),
+		HTTPAddr:  envOr("BACKEND_HTTP_ADDR", defaultHTTPAddr),
+		LogLevel:  envOr("BACKEND_LOG_LEVEL", defaultLogLevel),
+		Postgres:  pg,
+		Telemetry: tel,
 	}
 	if err := c.validate(); err != nil {
 		return Config{}, err
@@ -44,6 +76,12 @@ func (c Config) validate() error {
 	if c.HTTPAddr == "" {
 		return fmt.Errorf("config: BACKEND_HTTP_ADDR must not be empty")
 	}
+	if err := c.Postgres.Validate(); err != nil {
+		return fmt.Errorf("config: %w (set BACKEND_POSTGRES_DSN)", err)
+	}
+	if err := c.Telemetry.Validate(); err != nil {
+		return fmt.Errorf("config: %w", err)
+	}
 	return nil
 }
 
@@ -55,3 +93,31 @@ func envOr(key, fallback string) string {
 	}
 	return fallback
 }
+
+// envInt parses the environment variable named key as an int, returning
+// fallback when it is unset and an error when it is set but malformed.
+func envInt(key string, fallback int) (int, error) {
+	v := os.Getenv(key)
+	if v == "" {
+		return fallback, nil
+	}
+	n, err := strconv.Atoi(v)
+	if err != nil {
+		return 0, fmt.Errorf("config: %s: %w", key, err)
+	}
+	return n, nil
+}
+
+// envDuration parses the environment variable named key as a Go duration,
+// returning fallback when it is unset and an error when it is set but malformed.
+func envDuration(key string, fallback time.Duration) (time.Duration, error) {
+	v := os.Getenv(key)
+	if v == "" {
+		return fallback, nil
+	}
+	d, err := time.ParseDuration(v)
+	if err != nil {
+		return 0, fmt.Errorf("config: %s: %w", key, err)
+	}
+	return d, nil
+}