Extend Events and Admin pages with full feature set

Events page:
- Click-through links to /review/:id from event rows
- Cost estimation per review using model pricing data
- Export filtered events as CSV or JSON
- Live tail mode with pulsing indicator and new-row highlight
- Comparison mode: select 2 events for side-by-side delta analysis

Admin page:
- Time-range selector (1h/24h/7d/30d/All) filters all analytics
- Severity trends stacked area chart (Error/Warning/Info/Suggestion)
- Token cost dashboard with cumulative cost chart and monthly projection
- Category x Repository heatmap showing finding distribution patterns
- Period comparison mode splitting events into halves with delta indicators

Shared: add cost estimation utility (lib/cost.ts) using MODEL_PRESETS pricing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: haasonsaas

web/src/lib/cost.ts

@@ -0,0 +1,55 @@
+import { MODEL_PRESETS } from './models'
+import type { ReviewEvent } from '../api/types'
+
+// Parse price string like "$3", "$0.25", "free" into per-million-token cost
+function parsePricePerMillion(price: string): number {
+  if (price === 'free') return 0
+  const match = price.match(/\$?([\d.]+)/)
+  return match ? parseFloat(match[1]) : 0
+}
+
+// Build a lookup: normalized model fragment -> price per million tokens
+const priceLookup: [string[], number][] = MODEL_PRESETS.map(p => {
+  // Extract recognizable fragments from the preset ID
+  // e.g. "anthropic/claude-sonnet-4.6" -> ["claude-sonnet-4.6", "claude-sonnet", "sonnet-4.6"]
+  const parts = p.id.split('/')
+  const modelPart = parts[parts.length - 1].toLowerCase()
+  const fragments = [modelPart]
+  // Also store without version suffixes
+  const noVersion = modelPart.replace(/[-.]?\d+(\.\d+)*$/, '')
+  if (noVersion && noVersion !== modelPart) fragments.push(noVersion)
+  return [fragments, parsePricePerMillion(p.price)]
+})
+
+/** Estimate cost in USD for a review event based on total tokens and model pricing. */
+export function estimateCost(event: ReviewEvent): number {
+  const tokens = event.tokens_total ?? 0
+  if (tokens === 0) return 0
+
+  const modelLower = event.model.toLowerCase()
+  // Try to find a matching preset
+  for (const [fragments, pricePerM] of priceLookup) {
+    for (const frag of fragments) {
+      if (modelLower.includes(frag) || frag.includes(modelLower)) {
+        return (tokens / 1_000_000) * pricePerM
+      }
+    }
+  }
+
+  // Fallback: assume $1/M tokens (conservative middle ground)
+  return (tokens / 1_000_000) * 1
+}
+
+/** Format cost as a readable string */
+export function formatCost(usd: number): string {
+  if (usd === 0) return '$0'
+  if (usd < 0.001) return '<$0.001'
+  if (usd < 0.01) return `$${usd.toFixed(4)}`
+  if (usd < 1) return `$${usd.toFixed(3)}`
+  return `$${usd.toFixed(2)}`
+}
+
+/** Estimate total cost across multiple events */
+export function totalCost(events: ReviewEvent[]): number {
+  return events.reduce((sum, e) => sum + estimateCost(e), 0)
+}