chore(project): set the default port to 42776

Make 42776 the project's default listen port everywhere :8080 was the default:
.env.example, the Go config fallback, the Dockerfile (ENV/EXPOSE/healthcheck),
and the docs example. 42776 is the sum of the Unicode code points of 七夕
(七 U+4E03 = 19971, 夕 U+5915 = 22805).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/config/config.go

@@ -105,7 +105,7 @@ func Load() (*Config, error) {
 	}
 
 	cfg := &Config{
-		ListenAddr:    defaultStr("LISTEN_ADDR", ":8080"),
+		ListenAddr:    defaultStr("LISTEN_ADDR", ":42776"),
 		JWTSecret:     requireStr("JWT_SECRET"),
 		JWTAccessTTL:  parseDuration("JWT_ACCESS_TTL", "15m"),
 		JWTRefreshTTL: parseDuration("JWT_REFRESH_TTL", "720h"),