Move backfill horizon from a startup env var into the editable profile

SMARTRUN_BACKFILL_HORIZON_DAYS was a server-startup-only env var with no UI,
defaulting to 3 years -- so editing the unrelated "Rolling window" profile
field (for classification, not sync) had no effect on how far back Sync Now
reached. Backfill horizon is now Profile.BackfillHorizonDays, read fresh on
every Backfill call, with its own field on the Profile page.

backend/internal/config/config.go

@@ -28,7 +28,6 @@ type Config struct {
 	GarminTokenStore string
 
 	MinConfidence        float64
-	BackfillHorizonDays  int
 	IncrementalSyncEvery time.Duration
 }
 
@@ -42,7 +41,6 @@ func Load() (Config, error) {
 		GarminServerPath:     os.Getenv("MCP_GARMIN_SERVER"),
 		GarminTokenStore:     os.Getenv("GARMIN_TOKENSTORE"),
 		MinConfidence:        getEnvFloat("SMARTRUN_MIN_CONFIDENCE", 0.6),
-		BackfillHorizonDays:  getEnvInt("SMARTRUN_BACKFILL_HORIZON_DAYS", 3*365),
 		IncrementalSyncEvery: getEnvDuration("SMARTRUN_INCREMENTAL_SYNC_EVERY", 6*time.Hour),
 	}
 
@@ -71,15 +69,6 @@ func getEnvFloat(key string, def float64) float64 {
 	return def
 }
 
-func getEnvInt(key string, def int) int {
-	if v := os.Getenv(key); v != "" {
-		if n, err := strconv.Atoi(v); err == nil {
-			return n
-		}
-	}
-	return def
-}
-
 func getEnvDuration(key string, def time.Duration) time.Duration {
 	if v := os.Getenv(key); v != "" {
 		if d, err := time.ParseDuration(v); err == nil {