fix: address deep code review issues for RPM limiting

- Move IncrementRPM after Forward success to prevent phantom RPM
  consumption during account switch retries
- Add base_rpm input sanitization (clamp to 0-10000) in Create/Update
- Add WindowCost scheduling checks to legacy path sticky sessions
  (4 check sites + 4 prefetch sites), fixing pre-existing gap
- Clean up rpm_strategy/rpm_sticky_buffer when disabling RPM in
  BulkEditModal (JSONB merge cannot delete keys, use empty values)
- Add json.Number test cases to TestGetBaseRPM/TestGetRPMStickyBuffer
- Document TOCTOU race as accepted soft-limit design trade-off

backend/internal/service/account_rpm_test.go

@@ -1,6 +1,9 @@
 package service
 
-import "testing"
+import (
+	"encoding/json"
+	"testing"
+)
 
 func TestGetBaseRPM(t *testing.T) {
 	tests := []struct {
@@ -16,6 +19,7 @@ func TestGetBaseRPM(t *testing.T) {
 		{"string value", map[string]any{"base_rpm": "15"}, 15},
 		{"negative value", map[string]any{"base_rpm": -5}, 0},
 		{"int64 value", map[string]any{"base_rpm": int64(20)}, 20},
+		{"json.Number value", map[string]any{"base_rpm": json.Number("25")}, 25},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -103,6 +107,7 @@ func TestGetRPMStickyBuffer(t *testing.T) {
 		{"custom buffer=0 fallback to default", map[string]any{"base_rpm": 10, "rpm_sticky_buffer": 0}, 2},
 		{"custom buffer negative fallback", map[string]any{"base_rpm": 10, "rpm_sticky_buffer": -1}, 2},
 		{"custom buffer with float", map[string]any{"base_rpm": 10, "rpm_sticky_buffer": float64(7)}, 7},
+		{"json.Number base_rpm", map[string]any{"base_rpm": json.Number("10")}, 2},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {