sync: bring over remaining release/custom-0.1.115 changes

- Extract PublicSettingsInjectionPayload named struct with drift test
- Add channel_monitor_default_interval_seconds to SSR injection
- Add image_output_price to SupportedModelChip
- Simplify AppSidebar buildSelfNavItems (admins see available channels)
- Add gateway WARN logs for 503 no-available-accounts branches
- Wire ChannelMonitorRunner into provideCleanup for graceful shutdown
- Add migrations 130/131 (CC template userid fix + mimicry field cleanup)
- Clean up fork-only features (sora, claude max simulation, client affinity)
- Remove ~320 obsolete i18n keys
- Add codexUsage utility, WechatServiceButton, BulkEditAccountModal
- Tidy go.sum

backend/internal/pkg/ctxkey/ctxkey.go

@@ -55,4 +55,9 @@ const (
 
 	// ClaudeCodeVersion stores the extracted Claude Code version from User-Agent (e.g. "2.1.22")
 	ClaudeCodeVersion Key = "ctx_claude_code_version"
+
+	// IsSignatureRectifyRetry marks a retry request that was produced by the signature rectifier
+	// (strip or pool-replace). The harvester consults this flag to avoid ingesting signatures
+	// from retries, which would pollute the pool with signatures we ourselves injected.
+	IsSignatureRectifyRetry Key = "ctx_is_signature_rectify_retry"
 )