feat: add resource picker and multi-resource report generation

Replace manual resource ID entry with a searchable, filterable resource
picker that uses live WebSocket state. Support selecting multiple
resources (up to 50) for combined fleet reports.

Multi-resource PDFs include a cover page, fleet summary table with
aggregate health status, and condensed per-resource detail pages with
overlaid CPU/memory charts. Multi-resource CSVs include a summary
section followed by interleaved time-series data with resource columns.

New POST /api/admin/reports/generate-multi endpoint handles multi-resource
requests while the existing single-resource GET endpoint remains unchanged.

Also fixes resource ID validation regex to allow colons used in
VM/container IDs (e.g., "instance:node:vmid").

pkg/reporting/reporting.go

@@ -106,10 +106,31 @@ type DiskInfo struct {
 	WearLevel   int    // 0-100, percentage of life REMAINING (100 = healthy, 0 = end of life, -1 = unknown)
 }
 
+// MultiReportRequest defines the parameters for generating a multi-resource report.
+type MultiReportRequest struct {
+	Resources  []MetricReportRequest // One per resource, each with enrichment
+	Format     ReportFormat
+	Start      time.Time
+	End        time.Time
+	Title      string
+	MetricType string
+}
+
+// MultiReportData holds the data for multi-resource report generation.
+type MultiReportData struct {
+	Title       string
+	Start       time.Time
+	End         time.Time
+	GeneratedAt time.Time
+	Resources   []*ReportData // Reuse existing ReportData per resource
+	TotalPoints int
+}
+
 // Engine defines the interface for report generation.
 // This allows the enterprise version to provide PDF/CSV generation.
 type Engine interface {
 	Generate(req MetricReportRequest) (data []byte, contentType string, err error)
+	GenerateMulti(req MultiReportRequest) (data []byte, contentType string, err error)
 }
 
 var (