feat(display): limit /adults and /juniors to last N months by default

Show only the last MONTHS_TO_SHOW months (default 5) in the fee table columns
so the page fits on screen without horizontal scrolling. Reconciliation still
runs over the full month history so balances, credits, and debts are unaffected.
Set MONTHS_TO_SHOW=0 to show all months. Implemented in both Python and Go.

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

go/internal/config/config.go

@@ -64,6 +64,7 @@ type Config struct {
 	DriveTimeout     time.Duration
 	LogLevel         string
 	ServerAddr       string
+	MonthsToShow     int // show last N month columns; 0 means show all
 }
 
 // Load reads configuration from the environment, applying defaults that
@@ -87,6 +88,7 @@ func Load() Config {
 		DriveTimeout:     envDuration("DRIVE_TIMEOUT_SECONDS", 10),
 		LogLevel:         env("LOG_LEVEL", "INFO"),
 		ServerAddr:       env("SERVER_ADDR", ":8080"),
+		MonthsToShow:     envInt("MONTHS_TO_SHOW", 5),
 	}
 }
 
@@ -121,3 +123,12 @@ func envDuration(key string, defaultSeconds int) time.Duration {
 	}
 	return time.Duration(defaultSeconds) * time.Second
 }
+
+func envInt(key string, fallback int) int {
+	if v := os.Getenv(key); v != "" {
+		if n, err := strconv.Atoi(v); err == nil {
+			return n
+		}
+	}
+	return fallback
+}