feat: add cache control settings and token-based metrics

Settings:
- Add `alwaysPreserveClientCache` setting with modes: auto/always/never
- UI toggle in Dashboard > Settings > Routing tab
- Auto mode preserves cache_control for Claude Code clients with deterministic routing

Metrics:
- Track prompt cache token usage (input, cached, creation)
- Display cache reuse ratio (cached/input tokens)
- Breakdown by provider and routing strategy
- Shows tokens saved and estimated cost savings

API Endpoints:
- GET /api/settings/cache-metrics - retrieve metrics
- DELETE /api/settings/cache-metrics - reset metrics

Files:
- open-sse/utils/cacheControlPolicy.ts: CacheControlMetrics interface, trackCacheMetrics, updateCacheTokenMetrics
- open-sse/handlers/chatCore.ts: Track cache tokens from provider responses
- src/lib/db/settings.ts: Database functions for metrics persistence
- src/lib/cacheControlSettings.ts: Cached settings accessor
- src/app/(dashboard)/dashboard/settings/components/CacheStatsCard.tsx: Metrics dashboard UI
- tests/unit/*.test.mjs: Unit tests (41 tests pass)

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

src/shared/validation/settingsSchemas.ts

@@ -47,6 +47,8 @@ export const updateSettingsSchema = z.object({
   cliCompatProviders: z.array(z.string().max(100)).optional(),
   // Strip provider/model prefix at proxy layer (e.g. "openai/gpt-4" → "gpt-4")
   stripModelPrefix: z.boolean().optional(),
+  // Cache control preservation mode
+  alwaysPreserveClientCache: z.enum(["auto", "always", "never"]).optional(),
   // Custom CLI agent definitions for ACP
   customAgents: z
     .array(