Parallelize AI price check batches, increase batch size to 25

500-item catalog was making 50 sequential API calls, causing progressive rate-limit
throttling (explains "super slow towards the end") and ~$3 in credits.

- BatchSize: 10 → 25 (word limits are in place; 25 items × ~80 tokens ≈ 2000
  output tokens, well within MaxTokens=8192 — the original truncation cause)
- Run up to 3 batches concurrently via SemaphoreSlim(3) — independent API calls
  with no shared state, so no growing context issue
- For a 500-item catalog: 50 sequential calls → 20 calls in ~7 parallel waves,
  roughly 4× faster and 60% cheaper
- Dropped unused `costs` param from AnalyzeBatchAsync (system prompt has all costs)
- JS progress timing updated to reflect parallel waves

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

src/PowderCoating.Web/wwwroot/js/catalog-price-check.js

@@ -12,8 +12,10 @@
 
     // Estimate total seconds based on item count (roughly 12s per batch of 25, min 15s).
     function estimateDuration(itemCount) {
-        var batches = Math.max(1, Math.ceil(itemCount / 10));
-        return Math.max(15, batches * 10);
+        // 25 items/batch, up to 3 concurrent — wall time ≈ ceil(batches/3) × 12s
+        var batches = Math.max(1, Math.ceil(itemCount / 25));
+        var waves = Math.ceil(batches / 3);
+        return Math.max(15, waves * 12);
     }
 
     // Messages keyed to approximate progress milestones (0–100).