docs: refresh README to reflect MEP v2 quality & safety features

[amar-python]

Summary

Comprehensive README refresh documenting the current project state, including the code-quality work delivered in #10, #11, and #12.

Base branch / merge order: targets feature/schema-validation (like #10–#12) because the backend/ layout only exists there. Since this README documents features introduced in #10–#12, merge those three PRs first, then this one — otherwise a few referenced files (tools/verify_workflow_paths.py, docs/security/Rationale.md, .github/pull_request_template.md, backend/services/csv_parser.py) won't exist yet.

What changed

New: "MEP Backend — Reliability & Safety Features" section

• Migration run lifecycle state machine — diagram + state list (CREATED → UPLOADING → VALIDATING → READY → MIGRATING → COMPLETED, with ERROR/FAILED paths), verified against ALLOWED_TRANSITIONS in backend/database/models.py on the fix(data): robust CSV parser, migration run state machine, shell script hardening #11 branch
• CsvParser service — zero-byte/header-only rejection, ragged-row detection, UTF-8→latin-1 encoding fallback, suspicious-header flagging
• Production safety gate — ALLOW_SCHEMA_AUTO_CREATE + fail-fast validation when APP_ENV=production
• Typed API responses — Pydantic response_model on every endpoint; internal paths never leaked
• Shell script hardening — table-name regex + 63-char limit, --env validation, eval removal
• CI workflow path verification — tools/verify_workflow_paths.py usage
• Security suppression policy — links docs/security/Rationale.md
• Test instructions — how to run the backend suite, integration marker, legacy engine tests; notes growth from 38 → 100+ backend tests

Accuracy fixes to existing content

• All legacy engine commands updated from stale root paths to backend/migration/... (setup/deploy/test runners, csv_loader.sh, csv_utilise.sh, evals/runner.py, PowerShell runners, psql invocation) — matches the actual tree and the CI workflow's own path
• Engine "Repository Structure" tree re-rooted at backend/migration/ with an explanatory note; evals badge link fixed
• Repository layout diagram updated (tools/, backend/tests/, docs/security|ci, PR template)

Contributing

• Rewritten around the new PR template: Tests Added + Verified Workflow Paths checklists, response_model requirement, RunStatus lifecycle rule, suppression-documentation rule, correct test paths

Removed

• Stale "Acknowledgements" section — ~140 lines of leftover session prompts referencing an unrelated repository (amar-python/TestUploadtoGIT)

Verification

• Every backend/migration/... path referenced in the README was existence-checked against the working tree (all present)
• State machine diagram cross-checked against ALLOWED_TRANSITIONS on fix/data-integrity-safety
• Makefile csv-demo / csv-load targets confirmed to exist

Checklist

• Tests Added — docs-only change; no code touched (existing suites unaffected)
• Verified Workflow Paths — no workflow-referenced files moved

⚠️ Please review before merging — do not auto-merge. Recommended merge order: #7 → #10 → #11 → #12 → this PR.