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>

migrations/pdf_recognition.sql

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+-- migrations/pdf_recognition.sql
+-- 将 recognition_results 表重构为 JSON content schema
+-- 执行顺序：加新列 → 洗历史数据 → 删旧列
+
+-- Step 1: 新增 JSON 字段（保留旧字段，等数据迁移完再删）
+ALTER TABLE `recognition_results`
+  ADD COLUMN `meta_data` JSON DEFAULT NULL COMMENT '元数据 {"total_num":1}' AFTER `task_type`,
+  ADD COLUMN `content`   JSON DEFAULT NULL COMMENT '识别内容 JSON' AFTER `meta_data`;
+
+-- Step 2: 将旧列数据洗入新 JSON 字段
+-- 所有现有记录均为 FORMULA 类型（单页），meta_data.total_num = 1
+-- content 结构: {"latex":"...","markdown":"...","mathml":"...","mml":"..."}
+UPDATE `recognition_results`
+SET
+  `meta_data` = JSON_OBJECT('total_num', 1),
+  `content`   = JSON_OBJECT(
+    'latex',    IFNULL(`latex`, ''),
+    'markdown', IFNULL(`markdown`, ''),
+    'mathml',   IFNULL(`mathml`, ''),
+    'mml',      IFNULL(`mml`, '')
+  )
+WHERE `content` IS NULL;
+
+-- Step 3: 验证数据洗涤完成（应返回 0）
+-- SELECT COUNT(*) FROM `recognition_results` WHERE `content` IS NULL;
+
+-- Step 4: 删除旧字段
+ALTER TABLE `recognition_results`
+  DROP COLUMN `latex`,
+  DROP COLUMN `markdown`,
+  DROP COLUMN `mathml`,
+  DROP COLUMN `mml`;