test(backend): expand unit coverage; add docs & architecture diagrams

[MegumiinUwU]

Tests

• Add comprehensive MessagesService streaming/HITL/error-mapping tests (sendMessage stream + persistence, permission/clarification resume & event-merge, agent error mapping 502/503/504, malformed-line handling). messages.service.ts coverage 15% -> 98%.
• Cover the @res() streaming controller handlers, _addSession, and the null-title session mapper branch.
• Add message.findFirst to the shared Prisma mock.
• Enforce a coverage floor via jest coverageThreshold (lines/statements 80, branches 75, functions 85). Overall line coverage 54% -> 87%, 78 -> 105 tests.

Docs

• Fix inaccuracies in README and DOCUMENTATION (agent resume endpoints, handleEvent word re-chunking/pacing, DATABASE_URL resolution, method counts, corrected route table).
• Rename Docs/ -> docs/ and add docs/TESTING.md (coverage & methodology).

Diagrams

• Add diagrams/generate_diagrams.py (Kroki/PlantUML) plus 17 rendered, A4-friendly PNGs covering architecture, sequences, flows and data model.