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/model/pdf/request.go

@@ -0,0 +1,34 @@
+package pdf
+
+// CreatePDFRecognitionRequest 创建PDF识别任务
+type CreatePDFRecognitionRequest struct {
+	FileURL  string `json:"file_url" binding:"required"`
+	FileHash string `json:"file_hash" binding:"required"`
+	FileName string `json:"file_name" binding:"required"`
+	UserID   int64  `json:"user_id"`
+}
+
+// GetPDFTaskRequest URI 参数
+type GetPDFTaskRequest struct {
+	TaskNo string `uri:"task_no" binding:"required"`
+}
+
+// CreatePDFTaskResponse 创建任务响应
+type CreatePDFTaskResponse struct {
+	TaskNo string `json:"task_no"`
+	Status int    `json:"status"`
+}
+
+// PDFPageResult 单页结果
+type PDFPageResult struct {
+	PageNumber int    `json:"page_number"`
+	Markdown   string `json:"markdown"`
+}
+
+// GetPDFTaskResponse 查询任务状态和结果
+type GetPDFTaskResponse struct {
+	TaskNo     string          `json:"task_no"`
+	Status     int             `json:"status"`
+	TotalPages int             `json:"total_pages"`
+	Pages      []PDFPageResult `json:"pages"`
+}