Bus factor reduction: cross-training documentation + review process

[snowfox1003]

Problem

A single contributor owns ~90K LoC across 15 Django apps with little external review evidence. New contributors cannot safely change collectors without architecture context or enforced review.

Acceptance criteria

• Architecture overview document covering all 15 Django apps (purpose, boundaries, key entry points)
• .github/CODEOWNERS requiring at least one reviewer per PR
• core/ mapped to architecture owner; .github/workflows/ mapped to CI owner; per-app ownership per eval guidance
• At least two onboarding walkthrough outlines documented (targets: Leo, Jonathan)
• PR review process documented (linked from README or CONTRIBUTING.md)