revert: remove fork-only changes from release sync

Revert payment/wechat, sora/claude-max cleanup, fork-only migrations,
and cosmetic changes that were brought in by the release sync commit.
Keep only channel-monitor related improvements:
- PublicSettingsInjectionPayload named struct with drift test
- ChannelMonitorRunner graceful shutdown in wire
- image_output_price in SupportedModelChip
- Simplified buildSelfNavItems in AppSidebar
- Gateway WARN logs for 503 branches

backend/internal/pkg/ctxkey/ctxkey.go

@@ -55,9 +55,4 @@ 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"
 )