docs: rework repo for the end user (web UI) and trim developer docs

[briacSck]

Prepares the handoff to a non-technical macOS user who runs only the web UI.

User-facing:

• Rewrite README.md as a plain-language, web-UI, macOS walkthrough: one-time setup (install uv, make a Mistral key, fill .env), each grading run (start script → browser → upload zip → download), and privacy cleanup when done.
• Rewrite docs/data-policy.md in plain terms (what stays local, what is sent, check your Mistral retention setting, erase when done).
• Ship a ready-to-use config.yaml (un-ignored; holds no secrets) with the validated masking regexes, auto_orient, and min_native_dpi=140, so she never edits YAML; course is set per-batch in the upload form.

Foolproof setup:

• Load .env in-app via python-dotenv (web app + CLI), so the API key and the web login come from a file instead of shell env vars. Robust to special characters like '&' in the password (verified: .env-only login returns 200).
• Add .env.example, and double-click macOS scripts start.command / cleanup.command (executable bit + .gitattributes eol=lf so they run on a Mac).

Cleanup:

• Consolidate ARCHITECTURE + OPERATIONS + CHANGELOG into one concise MAINTAINERS.md.
• Delete deployment docs (docs/deploy.md, Dockerfile), the 586-line PRD (ocr-grading-tool-plan.md), and fold docs/runbook.md + docs/mistral-setup.md into the README. All recoverable from git history.

Tests: 59 passing (unchanged). No student data or trial artifacts are tracked.