feat: add PDF document recognition with 10-page pre-hook

- Migrate recognition_results table to JSON schema (meta_data + content),
  replacing flat latex/markdown/mathml/mml columns
- Add TaskTypePDF constant and update all formula read/write paths
- Add PDFRecognitionService using pdftoppm (Poppler) for CGO-free page
  rendering; limits processing to first 10 pages (pre-hook)
- Reuse existing downstream OCR endpoint (cloud.texpixel.com) for each
  page image; stores results as [{page_number, markdown}] JSON array
- Add Redis queue + distributed lock for PDF worker goroutine
- Add REST endpoints: POST /v1/pdf/recognition, GET /v1/pdf/recognition/:task_no
- Add .pdf to OSS upload file type whitelist
- Add migrations/pdf_recognition.sql for safe data migration

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

internal/storage/cache/pdf.go

@@ -0,0 +1,27 @@
+package cache
+
+import (
+	"context"
+	"strconv"
+)
+
+const (
+	PDFRecognitionTaskQueue = "pdf_recognition_queue"
+	PDFRecognitionDistLock  = "pdf_recognition_dist_lock"
+)
+
+func PushPDFTask(ctx context.Context, taskID int64) (int64, error) {
+	return RedisClient.LPush(ctx, PDFRecognitionTaskQueue, taskID).Result()
+}
+
+func PopPDFTask(ctx context.Context) (int64, error) {
+	result, err := RedisClient.BRPop(ctx, 0, PDFRecognitionTaskQueue).Result()
+	if err != nil {
+		return 0, err
+	}
+	return strconv.ParseInt(result[1], 10, 64)
+}
+
+func GetPDFDistributedLock(ctx context.Context) (bool, error) {
+	return RedisClient.SetNX(ctx, PDFRecognitionDistLock, "locked", DefaultLockTimeout).Result()
+}