feat: add auto-increment id, size/duration tracking, and soft cleanup to conversion_jobs

Rename the old CHAR(36) id to uuid (still used for public URLs and file
naming) and add a real auto-increment id as the primary key. Track
input/output file size and conversion duration per job. Cleanup no
longer deletes rows; it marks cleaned_at and skips already-cleaned
expired jobs on later runs.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

db/schema.sql

@@ -1,5 +1,6 @@
 CREATE TABLE IF NOT EXISTS conversion_jobs (
-  id CHAR(36) NOT NULL PRIMARY KEY,
+  id INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
+  uuid CHAR(36) NOT NULL,
   status ENUM('pending', 'processing', 'done', 'failed') NOT NULL DEFAULT 'pending',
   family VARCHAR(32) NOT NULL,
   source_format VARCHAR(16) NOT NULL,
@@ -9,11 +10,16 @@ CREATE TABLE IF NOT EXISTS conversion_jobs (
   output_path VARCHAR(255) NULL,
   input_mime_type VARCHAR(128) NOT NULL,
   output_mime_type VARCHAR(128) NULL,
+  input_size_bytes INT UNSIGNED NOT NULL,
+  output_size_bytes INT UNSIGNED NULL,
+  conversion_duration_seconds DECIMAL(10,3) NULL,
   error_message VARCHAR(255) NULL,
   error_log TEXT NULL,
   created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
   updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
   expires_at DATETIME NOT NULL,
+  cleaned_at DATETIME NULL DEFAULT NULL,
+  UNIQUE KEY uniq_uuid (uuid),
   INDEX idx_status (status),
   INDEX idx_expires_at (expires_at)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;