feat(gateway): add billing attribution block with cc_version fingerprint

Real Claude Code CLI always sends a 2-block system array:

  [0] {"type":"text", "text":"x-anthropic-billing-header: cc_version=X.Y.Z.{fp}; cc_entrypoint=cli; cch=00000;"}
  [1] {"type":"text", "text":"You are Claude Code...", "cache_control":{...}}

Before this commit, sub2api's mimicry path only produced block [1].
The missing billing block is one of the primary third-party detection
signals Anthropic uses for Claude-Code-scoped OAuth tokens.

New file gateway_billing_block.go ports the fingerprint algorithm
(byte-for-byte from Parrot cc_mimicry.py:compute_fingerprint):
pick chars at positions [4,7,20] of the first user text, then
`sha256(SALT + chars + cc_version)[:3]`.

  - claude/constants.go: CLICurrentVersion = "2.1.92" (must match UA)
  - gateway_billing_block.go: computeClaudeCodeFingerprint +
    buildBillingAttributionBlockJSON + extractFirstUserText
  - gateway_service.go: rewriteSystemForNonClaudeCode now emits both
    blocks in order; cch=00000 is filled in later by
    signBillingHeaderCCH in buildUpstreamRequest.

Downstream compat note: syncBillingHeaderVersion's regex
`cc_version=\d+\.\d+\.\d+` only matches the semver triple,
leaving the `.{fp}` suffix intact when rewriting in buildUpstreamRequest.

backend/internal/pkg/claude/constants.go

@@ -62,6 +62,11 @@ const APIKeyHaikuBetaHeader = BetaInterleavedThinking
 // 客户端缺省时统一使用 5m"，这样既不浪费 1h 缓存额度，也保留客户端自定义能力。
 const DefaultCacheControlTTL = "5m"
 
+// CLICurrentVersion 是 sub2api 当前对外伪装的 Claude Code CLI 版本号（三段 semver）。
+// 用于 billing attribution block 中的 cc_version=X.Y.Z.{fp} 前缀以及 fingerprint 计算。
+// 必须与 DefaultHeaders["User-Agent"] 中的版本号严格一致；不一致会被 Anthropic 判第三方。
+const CLICurrentVersion = "2.1.92"
+
 // FullClaudeCodeMimicryBetas 返回最"像"真实 Claude Code CLI 的完整 beta 列表，
 // 用于 OAuth 账号伪装成 Claude Code 时使用。
 // 顺序与真实 CLI 抓包一致。