feat(#373): prediction integration for cost estimation

- Create PredictionService for pre-task cost/token estimates
- Refresh common predictions on startup
- Integrate predictions into LLM telemetry tracker
- Add GET /api/telemetry/estimate endpoint
- Graceful degradation when no prediction data available
- Add unit tests for prediction service

Refs #373

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

apps/api/src/llm/llm-telemetry-tracker.service.spec.ts

@@ -333,12 +333,13 @@ describe("LlmTelemetryTrackerService", () => {
       service.trackLlmCompletion(baseParams);
 
       // claude-sonnet-4-5: 150 * 3 + 300 * 15 = 450 + 4500 = 4950
-      const expectedCost = 4950;
+      const expectedActualCost = 4950;
 
       expect(mockTelemetryService.eventBuilder?.build).toHaveBeenCalledWith(
         expect.objectContaining({
-          estimated_cost_usd_micros: expectedCost,
-          actual_cost_usd_micros: expectedCost,
+          // Estimated values are 0 when no PredictionService is injected
+          estimated_cost_usd_micros: 0,
+          actual_cost_usd_micros: expectedActualCost,
         }),
       );
     });
@@ -437,8 +438,9 @@ describe("LlmTelemetryTrackerService", () => {
         expect.objectContaining({
           actual_input_tokens: 50,
           actual_output_tokens: 100,
-          estimated_input_tokens: 50,
-          estimated_output_tokens: 100,
+          // Estimated values are 0 when no PredictionService is injected
+          estimated_input_tokens: 0,
+          estimated_output_tokens: 0,
         }),
       );
     });