Add persistent multi-page scan sessions

[fgrfn]

Summary

• Persist multi-page scan sessions server-side with SQLite-backed recovery after restarts.
• Add interactive and automatic ADF modes, page previews, reorder/rotate/delete, resume and cancellation.
• Add blank-page removal, contrast/crop optimization, OCR and optional PDF/A-2 output.
• Add typed frontend helpers, unit and Playwright tests, CI coverage, Docker dependencies and architecture documentation.

Why

Multi-page scans were previously held in browser memory and were fragile across reloads or service restarts. The new session workflow keeps captured pages safely on the server until the final PDF is created and supports both flatbed and ADF workflows.

Validation

• Python test suite: 25 passed
• Frontend unit tests: 3 passed
• Svelte check: 0 errors and 0 warnings
• Playwright multi-page flow: 1 passed
• Vite production build: passed
• git diff --check: passed

Docker is unavailable in the local environment, so the container build was not run locally; CI contains the container smoke test.