docs: align README and docs with runtime contracts (live payload, API, diagrams, config)

[AyushmanRaha]

Motivation

• Perform a documentation truth-audit so README and docs match the actual FastAPI backend, runtime settings, and repository layout.
• Fix known mismatches (the /live response shape, max_dim defaults/validation, architecture diagrams, and configuration environment variables) so user-facing docs are accurate and verifiable.
• Keep wording conservative where behavior is implementation-dependent (runtime numeric fields, ONNX optionality, Redis fallback).
• Make the diagrams and API reference explicitly reflect current public routes, aliases, and diagnostic endpoints.

Description

• Updated README.md to document the actual /live payload (adds runtime-dependent pid, timestamp, and uptime_seconds) and to replace the lightweight testing snippet with the current docs/validation commands such as python scripts/validate_docs.py and scripts/ci.sh docs-contract.
• Adjusted docs/api-reference.md to clarify that max_dim is optional at the route level (route default None) and that validation enforces 64 <= max_dim <= DEPTHLENS_MAX_DIM (configured default 1536).
• Revised architecture artifacts: updated the Mermaid diagram in docs/system-architecture.md and the static SVG docs/architecture/depthlens-system-architecture.svg to include /live, /ready, /health, /compare aliases, /api/* aliases, detection endpoints, observability/metrics/cache routes, and the COCO detector node.
• Expanded docs/configuration.md to surface route timeout/CORS variables, benchmark/ONNX diagnostic env vars, and clarified that DEPTHLENS_CACHE_BACKEND=memory is a CI/setup hint while the runtime attempts Redis then falls back to in-memory.
• Fixed a grammatical sentence in docs/project-structure.md and added the resource-path-contract.md entry to the documented docs tree.

Testing

• Ran python scripts/validate_docs.py which passed (documentation contract check succeeded).
• Ran scripts/ci.sh docs-contract which executed the docs contract job and returned success in this environment.
• Compiled key modules with python -m py_compile backend/api/live.py backend/api/routes.py backend/main.py backend/app.py scripts/validate_docs.py which completed without syntax errors.
• Ran npm --prefix electron-app test which executed the Electron frontend contract tests and the included resource/security checks and they passed in this environment.
• Attempted python -m pytest -q backend/tests/test_lightweight_live.py backend/tests/test_routes.py backend/tests/test_refactor_contract.py, but collection failed with ModuleNotFoundError: No module named 'fastapi' because backend test dependencies are not installed; per constraints no dependency installation was performed, so pytest was not executed to completion.

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Codex Task