feat(sponsor-panel): add Prometheus metrics for SRE observability

[Xe]

Summary

• Add per-handler HTTP request duration histograms (sponsor_panel_http_request_duration_seconds) with handler, method, and status code labels
• Add per-handler failure/success counters (sponsor_panel_http_requests_total) and in-flight gauges (sponsor_panel_http_requests_in_flight)
• Add response size histograms (sponsor_panel_http_response_size_bytes)
• Add sponsor sync metrics: duration histogram, success/error counter, active sponsors gauge
• Add OAuth login counters by provider and result (success, state mismatch, token exchange error, upsert error)
• Add session decode error counter

Metrics exposed

Metric	Type	Labels
sponsor_panel_http_request_duration_seconds	Histogram	handler, method, code
sponsor_panel_http_requests_total	Counter	handler, method, code
sponsor_panel_http_requests_in_flight	Gauge	handler
sponsor_panel_http_response_size_bytes	Histogram	handler
sponsor_panel_sync_duration_seconds	Histogram	—
sponsor_panel_sync_total	Counter	result
sponsor_panel_active_sponsors	Gauge	—
sponsor_panel_oauth_total	Counter	provider, result
sponsor_panel_session_errors_total	Counter	—

Test plan

• go build ./cmd/sponsor-panel/ passes
• Verify /metrics endpoint exposes new metrics after deploying
• Confirm handler labels match expected route names in Prometheus/Grafana