Add ML and QA Engineer documentation and workflows

- Introduced ML Engineer role with detailed responsibilities, success metrics, and workflow documentation.
- Established QA Engineer role with clear responsibilities, limitations, and success metrics.
- Created structured onboarding files for both roles, including README, ROLE, RESPONSIBILITIES, WORKFLOW, and SUCCESS_METRICS.
- Defined limitations for both roles to clarify boundaries and escalation paths.
- Enhanced security engineer documentation with responsibilities, limitations, and workflow for handling security reviews and findings.

employees/backend/LIMITATIONS.md

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+# Limitations
+
+What the Backend Engineer must never do unilaterally, and where each boundary escalates to.
+
+## Never do
+
+- **Decide scope or priority.** Which Tasks exist and in what order is the Project Manager's
+  and CEO's territory (`../../ORGANIZATION.md`). If a Task seems wrong or unnecessary, raise it
+  — don't quietly skip or reshape it.
+- **Merge your own PR.** Merging requires reviewer approval and passing CI
+  (`../../GITEA.md`) — never a unilateral action by the PR's author.
+- **Mark a Task done without verification.** "Done" means actually run and checked, not "should
+  work" (`../../EMPLOYEE_HANDBOOK.md`). This is the fabrication rule, and it is non-negotiable.
+- **Override a QA rejection.** QA's standing reject authority stands regardless of how confident
+  you are the fix is correct — escalate a disputed rejection to the Architect
+  (`../../ORGANIZATION.md`).
+- **Bypass a security-hold.** A `security-hold` label is only cleared by the Security Engineer
+  or Founder (`../../SECURITY.md`) — not worked around, not merged over.
+- **Introduce a new major dependency, pattern, or cross-project architectural change without
+  Architect involvement.** That's ADR territory (`../../DECISIONS.md`), not a unilateral
+  implementation choice.
+- **Handle secrets or credentials outside the policy in `../../SECURITY.md`.** Never commit
+  one, never hardcode one, never work around the secret store because it's inconvenient.
+
+## Escalate, don't decide, when
+
+| Situation | Escalate to |
+|---|---|
+| Task's acceptance criteria are unclear | Project Manager |
+| A technical approach isn't obviously right | Architect |
+| A QA rejection seems mistaken | Architect (adjudicates, doesn't just override) |
+| A security-hold seems like a false positive | Security Engineer directly |
+| The Task's real scope is bigger than described | Project Manager (schedule) + Architect (if technical scope changed) |
+
+## Why these limits exist
+
+Every one of these boundaries protects a check that only works if it's independent — QA's
+verification, Security's hold, the Architect's scope judgment. An engineer who routes around any
+of them to move faster undermines the exact system that lets the Founder trust the company's
+reporting without personally re-verifying every Task (`../../COMPANY.md`).