feat(billing): 统一计价与候选冻结估算

新增 effective-pricing-v2、9 位十进制定点计算、显式免费校验和结构化缺价错误。估价与生产预处理覆盖全部可用候选，并按最大候选费用冻结；规则优先级为平台模型、平台、基准模型。\n\n同步计价契约和 OpenAPI，补充 Token 参数别名、视频五秒向上取整及缺价回归测试。\n\n验证：go test ./...、pnpm openapi、Web 测试与构建通过。

apps/api/docs/swagger.json

@@ -6017,6 +6017,12 @@
                         "schema": {
                             "$ref": "#/definitions/httpapi.ErrorEnvelope"
                         }
+                    },
+                    "503": {
+                        "description": "Service Unavailable",
+                        "schema": {
+                            "$ref": "#/definitions/httpapi.ErrorEnvelope"
+                        }
                     }
                 }
             }
@@ -11227,6 +11233,14 @@
         "httpapi.PricingEstimateResponse": {
             "type": "object",
             "properties": {
+                "candidateCount": {
+                    "type": "integer",
+                    "example": 2
+                },
+                "currency": {
+                    "type": "string",
+                    "example": "resource"
+                },
                 "items": {
                     "type": "array",
                     "items": {
@@ -11234,9 +11248,25 @@
                         "additionalProperties": true
                     }
                 },
+                "pricingVersion": {
+                    "type": "string",
+                    "example": "effective-pricing-v2"
+                },
+                "requestFingerprint": {
+                    "type": "string",
+                    "example": "76ef6a537de8e71bd1ca93acadc078dbdbfa9f17e45224e4f9df59f535d2886f"
+                },
+                "reservationAmount": {
+                    "type": "number",
+                    "example": 2.75
+                },
                 "resolver": {
                     "type": "string",
-                    "example": "effective-pricing-v1"
+                    "example": "effective-pricing-v2"
+                },
+                "totalAmount": {
+                    "type": "number",
+                    "example": 1.25
                 }
             }
         },
@@ -13966,6 +13996,12 @@
                     "type": "object",
                     "additionalProperties": {}
                 },
+                "effectiveFrom": {
+                    "type": "string"
+                },
+                "effectiveTo": {
+                    "type": "string"
+                },
                 "formulaConfig": {
                     "type": "object",
                     "additionalProperties": {}
@@ -13973,6 +14009,9 @@
                 "id": {
                     "type": "string"
                 },
+                "isFree": {
+                    "type": "boolean"
+                },
                 "metadata": {
                     "type": "object",
                     "additionalProperties": {}
@@ -14033,10 +14072,19 @@
                     "type": "object",
                     "additionalProperties": {}
                 },
+                "effectiveFrom": {
+                    "type": "string"
+                },
+                "effectiveTo": {
+                    "type": "string"
+                },
                 "formulaConfig": {
                     "type": "object",
                     "additionalProperties": {}
                 },
+                "isFree": {
+                    "type": "boolean"
+                },
                 "metadata": {
                     "type": "object",
                     "additionalProperties": {}