feat(claude): add ttl to cache_control with default 5m

Real Claude CLI traffic sends cache_control as
`{"type":"ephemeral","ttl":"1h"}`. Our previous payload only
sent `{"type":"ephemeral"}`, which is a bytewise mismatch with
the official CLI and one more third-party detection signal.

Policy: client-provided ttl is always passed through unchanged.
Proxy-generated cache_control blocks default to 5m (vs Parrot's 1h)
to avoid burning the 1h cache budget on automatic breakpoints while
still aligning with the `ttl` field being present.

  - claude/constants.go: DefaultCacheControlTTL = "5m"
  - apicompat/types.go: new AnthropicCacheControl type with TTL field;
    AnthropicTool gains optional CacheControl pointer so the mimicry
    path can attach a cache breakpoint to tools[-1] later.
  - service/gateway_service.go: anthropicCacheControlPayload gains TTL;
    marshalAnthropicSystemTextBlock and rewriteSystemForNonClaudeCode
    emit ttl=5m by default.

backend/internal/pkg/claude/constants.go

@@ -57,6 +57,11 @@ const APIKeyBetaHeader = BetaClaudeCode + "," + BetaInterleavedThinking + "," +
 // APIKeyHaikuBetaHeader Haiku 模型在 API-key 账号下使用的 anthropic-beta header（不包含 oauth / claude-code）
 const APIKeyHaikuBetaHeader = BetaInterleavedThinking
 
+// DefaultCacheControlTTL 是网关代理为自己生成的 cache_control 块默认使用的 ttl。
+// 真实 Claude Code CLI 当前使用 "1h"，但本仓策略是"客户端透传 ttl 优先；
+// 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"
+
 // 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。