docs: ADR 0001 + Python backend scaffolding for single-backend migration (Phase 0)

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Summary

Kicks off a phased architecture simplification: collapse the three-tier polyglot stack (React → .NET gateway → Python engine) into a two-service architecture — React frontend talking directly to a Python FastAPI single backend — and delete the .NET gateway.

This PR is Phase 0: the decision record plus backend scaffolding. It is additive with zero runtime behavior change — the empty routers add no endpoints, and .NET still serves the frontend exactly as before.

Why

A full-stack review (CODEBASE_REVIEW.md) confirmed the .NET Composite/Mosaic/Mast/Analysis/SemanticSearch services are HTTP proxies to the Python engine, which already owns those domains end-to-end. The gateway's only non-duplicated responsibilities are auth, MongoDB persistence (just 2 collections), job tracking + SignalR, and a storage abstraction Python already has. Astropy must stay in Python, so the natural simplification is to make Python the single backend.

Outcome of the full migration: one backend instead of two, ~44k fewer lines of C# (incl. tests), no snake_case ↔ camelCase boundary, and far fewer containers/compose files. The system stays shippable after every phase (strangler-fig).

Changes Made

• ADR docs/architecture/adr/0001-collapse-to-python-single-backend.md — context, decision, consequences, and the 8-phase roadmap. Linked from the architecture index and mkdocs nav.
• Scaffolding packages under processing-engine/app/: auth/, db/, library/, jobs/. Empty APIRouters wired into main.py to establish the import graph and OpenAPI tags ahead of Phases 1–3.
• requirements.txt — adds motor (async MongoDB), pyjwt, passlib[bcrypt], bcrypt for the upcoming persistence + auth phases.
• Docs — AGENTS.md and docs/architecture/system-overview.md note the in-progress target architecture and where new backend code lives.

Test Plan

• New Python files parse cleanly (ast.parse); router wiring is mechanically identical to the five existing routers.
• No endpoints added — empty routers are valid to include; OpenAPI gains only the new tags.
• Full docker exec jwst-processing python -m pytest in CI (fastapi/deps run in Docker, not in this environment).

Documentation Checklist

• docs/architecture/ (new ADR + system-overview note)
• No other doc updates needed for Phase 0

Tech Debt Impact

• Existing tech debt reduced — establishes the path to remove the redundant .NET proxy tier and the duplicated storage abstraction.

Risk & Rollback

• Risk: Minimal. Additive only; no behavior change. New deps are unused until Phase 1.
• Rollback: Revert this commit — removes the scaffolding packages, deps, and docs with no impact on the running system.

Follow-up phases

Each lands as its own PR: (1) Mongo + Auth, (2) Library/persistence, (3) Jobs + WebSocket, (4) MAST/discovery wiring, (5) frontend cutover, (6) delete .NET, (7) infra simplification, (8) in-tier god-file cleanup.

https://claude.ai/code/session_01MWMesVhoX13SVmCcgjWrDF

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Generated by Claude Code

Note: recreated from #1571, which was auto-closed when its head branch was renamed from claude/review-architecture-simplify-AlLg2 (disallowed prefix) to refactor/single-backend-phase-0. Includes a ruff I001 import-sort fix on the three scaffolding routers.