feat: add rectangle/page UI, backend full-page tighten fix, and expanded docs

[theodubus]

Motivation

• Clarify product behavior and limits and provide packaging guidance via updated documentation for maintainers and non-technical users.
• Add UI affordances for page-level and rectangle redactions so operators can draw and manage rectangle rules in the viewer.
• Ensure full-page or very tall rectangle redactions are not over-tightened by the backend so intended large redactions actually remove the content.
• Improve end-to-end correctness by sending stricter sanitation options from the frontend when rectangle/page redactions are used.

Description

• Documentation: rewrite README.md to explain redaction theory, feature set, limits (including flattened/scanned PDFs), media placeholders and roadmap, add CONTRIBUTING.md, and add docs/DEPLOYMENT.md with Docker and packaging guidance, and clarify docs/SECURITY.md invariants and recommendations.
• Backend: change _tighten_rect_vertical in backend/app/redaction.py to skip aggressive vertical tightening for large rects (h > 24.0) so full-page/very-tall rectangles are preserved.
• Tests: add an integration test backend/tests/test_redact_rectangles.py::test_redact_rectangles_full_page_rect_redacts_target that posts a full-page rectangle and asserts the audit pass and that the secret is removed.
• Frontend: implement rectangle drawing and page rules and wire them through the app by updating frontend/src/App.tsx, frontend/src/components/PdfViewer.tsx, frontend/src/components/Rules/*, frontend/src/types/uiRules.ts, frontend/src/api.ts, locales, and styles; the UI now supports drawing rectangles, adding page rules, preview badges, tracking current page and page sizes, and sending apply_images, apply_graphics, sanitize_metadata, remove_annotations, and remove_attachments in the /api/redact/apply payload when applicable.

Testing

• Ran git diff --check which passed with no whitespace/patch issues.
• Performed keyword scans (rg) and manual doc sanity checks on the changed docs which passed.
• No backend or frontend test suites were executed in this rollout; the new pytest integration test test_redact_rectangles_full_page_rect_redacts_target was added but not run here.

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Codex Task