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.go

@@ -13,17 +13,9 @@ import (
 	"github.com/rs/zerolog"
 	"github.com/rs/zerolog/log"
 	"gopkg.in/lumberjack.v2"
-)
 
-// Config holds logging configuration, mirrors config.LoggingConfig.
-type Config struct {
-	Level      string
-	File       string
-	MaxSizeMB  int
-	MaxBackups int
-	MaxAgeDays int
-	Compress   bool
-}
+	"github.com/fujin/anthropic-proxy/internal/config"
+)
 
 // Setup initializes the global zerolog logger.
 // - File set: JSON → lumberjack rotating file
@@ -31,7 +23,7 @@ type Config struct {
 // - File empty + not TTY: JSON → stderr (for systemd journal)
 // Extra writers (e.g., OTLP log bridge) are added via io.MultiWriter so logs
 // are written to both the primary destination and any extra writers.
-func Setup(cfg Config, extraWriters ...io.Writer) zerolog.Logger {
+func Setup(cfg config.LoggingConfig, extraWriters ...io.Writer) zerolog.Logger {
 	// Parse log level
 	level, err := zerolog.ParseLevel(cfg.Level)
 	if err != nil || cfg.Level == "" {