feat(observability): configurable logging, request ids and error logs

- internal/logging: LOG_LEVEL/LOG_FORMAT parsing and slog handler
- request_id middleware (X-Request-ID or generated, echoed) + debug access log
- httpx registers errors; middleware logs internal causes with request_id
- health excluded, objects/sse debug-only, no query/body/secrets in logs
- new observability capability spec; README/.env.example docs

backend/internal/config/config.go

@@ -5,6 +5,8 @@ import (
 	"os"
 	"strconv"
 	"time"
+
+	"galo/internal/logging"
 )
 
 type Postgres struct {
@@ -63,6 +65,11 @@ type Database struct {
 	SQLitePath string
 }
 
+type Log struct {
+	Level  string
+	Format string
+}
+
 type Config struct {
 	Postgres Postgres
 	S3       S3
@@ -72,6 +79,7 @@ type Config struct {
 	Server   Server
 	SSE      SSE
 	Worker   Worker
+	Log      Log
 }
 
 func Load() (*Config, error) {
@@ -125,6 +133,15 @@ func Load() (*Config, error) {
 		return nil, fmt.Errorf("invalid WORKER_PENDING_TTL: %w", err)
 	}
 
+	logLevel := getEnv("LOG_LEVEL", "info")
+	if _, err := logging.ParseLevel(logLevel); err != nil {
+		return nil, err
+	}
+	logFormat := getEnv("LOG_FORMAT", "json")
+	if _, err := logging.ParseFormat(logFormat); err != nil {
+		return nil, err
+	}
+
 	cfg := &Config{
 		Postgres: Postgres{
 			Host:     getEnv("PG_HOST", "pg.localnet"),
@@ -169,6 +186,10 @@ func Load() (*Config, error) {
 			Batch:      workerBatch,
 			PendingTTL: workerPendingTTL,
 		},
+		Log: Log{
+			Level:  logLevel,
+			Format: logFormat,
+		},
 	}
 
 	switch cfg.Database.Backend {

backend/internal/config/config_test.go

@@ -132,3 +132,47 @@ func TestLoadDatabaseBackend(t *testing.T) {
 		}
 	})
 }
+
+func TestLoadLog(t *testing.T) {
+	t.Run("defaults", func(t *testing.T) {
+		setRequiredEnv(t)
+		t.Setenv("LOG_LEVEL", "")
+		t.Setenv("LOG_FORMAT", "")
+		cfg, err := Load()
+		if err != nil {
+			t.Fatalf("load: %v", err)
+		}
+		if cfg.Log.Level != "info" || cfg.Log.Format != "json" {
+			t.Fatalf("log = %+v, want info/json", cfg.Log)
+		}
+	})
+
+	t.Run("custom values", func(t *testing.T) {
+		setRequiredEnv(t)
+		t.Setenv("LOG_LEVEL", "debug")
+		t.Setenv("LOG_FORMAT", "text")
+		cfg, err := Load()
+		if err != nil {
+			t.Fatalf("load: %v", err)
+		}
+		if cfg.Log.Level != "debug" || cfg.Log.Format != "text" {
+			t.Fatalf("log = %+v, want debug/text", cfg.Log)
+		}
+	})
+
+	t.Run("invalid level", func(t *testing.T) {
+		setRequiredEnv(t)
+		t.Setenv("LOG_LEVEL", "trace")
+		if _, err := Load(); err == nil {
+			t.Fatal("expected error for invalid LOG_LEVEL")
+		}
+	})
+
+	t.Run("invalid format", func(t *testing.T) {
+		setRequiredEnv(t)
+		t.Setenv("LOG_FORMAT", "xml")
+		if _, err := Load(); err == nil {
+			t.Fatal("expected error for invalid LOG_FORMAT")
+		}
+	})
+}