refactor: modularize codebase — deduplicate, extract, clean up

- Unify duplicate uTLS transports into shared internal/transport package
- Extract shared version constant into internal/version
- Move LoadDefaultCredentials from config to auth (remove config→auth import)
- Deduplicate handler.go: extract telemetry/error helpers (324→268 lines)
- Break up main.go::run() into initCredential/initEmbedded
- Eliminate logging.Config duplication (use config.LoggingConfig directly)
- Extract logWriter to embedded/log.go, SSE fixtures to consts in sniff.go
- Use uTLS client for usage polling (consistent TLS fingerprint)
- Handle sjson.SetBytes errors in sanitize.go instead of silently swallowing
- Document reverse-engineered magic values in billing.go
- Unexport Credential.CooldownUntil (internal state)
- Replace hardcoded auth bypass paths with map in server.go

internal/logging/logging_test.go

@@ -9,6 +9,8 @@ import (
 	"testing"
 
 	"github.com/rs/zerolog"
+
+	"github.com/fujin/anthropic-proxy/internal/config"
 )
 
 func TestRedactHeaders(t *testing.T) {
@@ -177,7 +179,7 @@ func TestSetup_WithFile(t *testing.T) {
 	dir := t.TempDir()
 	logFile := filepath.Join(dir, "test.log")
 
-	logger := Setup(Config{
+	logger := Setup(config.LoggingConfig{
 		Level:      "debug",
 		File:       logFile,
 		MaxSizeMB:  10,
@@ -191,7 +193,7 @@ func TestSetup_WithFile(t *testing.T) {
 
 func TestSetup_WithoutFile(t *testing.T) {
 	// File empty — should use console or stderr mode depending on TTY
-	logger := Setup(Config{
+	logger := Setup(config.LoggingConfig{
 		Level: "warn",
 	})
 
@@ -201,13 +203,13 @@ func TestSetup_WithoutFile(t *testing.T) {
 
 func TestSetup_DefaultLevel(t *testing.T) {
 	// Empty level should default to info
-	logger := Setup(Config{})
+	logger := Setup(config.LoggingConfig{})
 	_ = logger // verify no panic
 }
 
 func TestSetup_InvalidLevel(t *testing.T) {
 	// Invalid level should default to info
-	logger := Setup(Config{Level: "not-a-level"})
+	logger := Setup(config.LoggingConfig{Level: "not-a-level"})
 	_ = logger // verify no panic
 }