feat(jobs): introduce extracting_pages status and update job progress handling

- Added a new job status 'extracting_pages' to represent the first sub-phase of thumbnail generation.
- Updated the database schema to include a timestamp for when thumbnail generation starts.
- Enhanced job progress components to handle the new status, including UI updates for displaying progress and status labels.
- Refactored job-related logic to accommodate the two-phase process: extracting pages and generating thumbnails.
- Adjusted SQL queries and job detail responses to include the new fields and statuses.

This change improves the clarity of job processing states and enhances user feedback during the thumbnail generation process.

infra/migrations/0018_add_extracting_pages_status.sql

@@ -0,0 +1,7 @@
+-- Migration: Add status 'extracting_pages' for the first sub-phase of thumbnail generation
+-- Phase 1 (extracting_pages): extract raw first-page image from archive, store as-is
+-- Phase 2 (generating_thumbnails): resize and encode as WebP
+ALTER TABLE index_jobs
+  DROP CONSTRAINT IF EXISTS index_jobs_status_check,
+  ADD CONSTRAINT index_jobs_status_check
+    CHECK (status IN ('pending', 'running', 'extracting_pages', 'generating_thumbnails', 'success', 'failed', 'cancelled'));

infra/migrations/0019_add_generating_thumbnails_started_at.sql

@@ -0,0 +1,5 @@
+-- Add timestamp for Phase 2b (generating_thumbnails) so we can show separate durations:
+-- Phase 2a: phase2_started_at → generating_thumbnails_started_at (extracting_pages)
+-- Phase 2b: generating_thumbnails_started_at → finished_at
+ALTER TABLE index_jobs
+  ADD COLUMN IF NOT EXISTS generating_thumbnails_started_at TIMESTAMPTZ;