Update backend AGENTS.md and README.md to SOTA 2026 standards

[google-labs-jules]

Updated backend documentation (AGENTS.md and README.md) to reflect the 'State of the Art 2026' standards. This includes adopting uv for package management, emphasizing file-scoped commands for performance, detailing the observability stack (JSON logging, SystemMonitor), and clarifying project structure and coding boundaries for AI agents.

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PR created automatically by Jules for task 8917670091444642142 started by @zarvent

[google-labs-jules]

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