feat: scaffold app skeleton with config, ULID, health endpoints, compose stack (phase 1)

- internal .env loader (real env wins) + strictly validated config struct
- ULID package: 48-bit ms timestamp + 80-bit crypto randomness, sortable
- HTTP server with recovery/request-id/trusted-proxy/access-log middleware
- /healthz, /readyz (pluggable checkers), /metricsz (counter registry)
- multi-stage Dockerfile (alpine, non-root) + compose with healthchecks,
  app bound to 127.0.0.1:8787, mongo unpublished, graceful 15s shutdown

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/config/env_test.go

@@ -0,0 +1,66 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestLoadDotEnv(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, ".env")
+	content := `# full-line comment
+PLAIN=hello
+INLINE=value # trailing comment
+QUOTED="quoted # not a comment"
+SINGLE='single quoted'
+export EXPORTED=yes
+EMPTY=
+SPACED =  padded value
+`
+	if err := os.WriteFile(path, []byte(content), 0o600); err != nil {
+		t.Fatal(err)
+	}
+
+	t.Setenv("PLAIN", "preexisting") // real env must win
+	for _, k := range []string{"INLINE", "QUOTED", "SINGLE", "EXPORTED", "EMPTY", "SPACED"} {
+		os.Unsetenv(k)
+		t.Cleanup(func() { os.Unsetenv(k) })
+	}
+
+	if err := LoadDotEnv(path); err != nil {
+		t.Fatalf("LoadDotEnv: %v", err)
+	}
+
+	want := map[string]string{
+		"PLAIN":    "preexisting",
+		"INLINE":   "value",
+		"QUOTED":   "quoted # not a comment",
+		"SINGLE":   "single quoted",
+		"EXPORTED": "yes",
+		"EMPTY":    "",
+		"SPACED":   "padded value",
+	}
+	for k, v := range want {
+		if got := os.Getenv(k); got != v {
+			t.Errorf("%s = %q, want %q", k, got, v)
+		}
+	}
+}
+
+func TestLoadDotEnvMissingFileIsOK(t *testing.T) {
+	if err := LoadDotEnv(filepath.Join(t.TempDir(), "absent")); err != nil {
+		t.Fatalf("missing file should not error, got %v", err)
+	}
+}
+
+func TestLoadDotEnvMalformed(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, ".env")
+	if err := os.WriteFile(path, []byte("NOT A KV LINE\n"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	if err := LoadDotEnv(path); err == nil {
+		t.Fatal("want error for malformed line")
+	}
+}