Slice 1: scaffolding + propose-to-super-draft vertical

Brings the §1 bot wrapper, the §4 cache (webhook + reconciler), the
§5 schema (six numbered migrations), Gitea OAuth + §6 user
provisioning, the §7 catalog left pane, and the propose-to-merge
vertical: propose modal opens an idea PR against the meta repo, an
owner merges from the pending-idea view, the cache picks it up via
webhook or reconciler sweep, and the catalog renders the new
super-draft.

Per §1 the bot is the only Git writer; every commit, branch
creation, and PR merge carries the §6.5 On-behalf-of: trailer and
an `actions` audit row. Per §4 the cache is never written from a
user action — it's webhook+reconciler only.

Covered by `backend/tests/test_propose_vertical.py` against an
in-process Gitea simulator.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/requirements.txt

@@ -0,0 +1,10 @@
+fastapi>=0.115
+uvicorn[standard]>=0.32
+httpx>=0.27
+python-dotenv>=1.0
+itsdangerous>=2.2
+pydantic>=2.9
+anthropic>=0.39
+google-generativeai>=0.8
+openai>=1.50
+PyYAML>=6.0