Slice 2: the §8 active-RFC view in full

Per the §19.1 brief: the three-column shape (§8.1) opens on main
in discuss mode (§8.2), supports the §8.3 discuss-vs-contribute
flip on non-main branches, hosts §8.4's per-branch chat with AI
participation (§18's <change> protocol → §8.14 changes rows), the
§8.8 change-card panel with §8.9 accept/decline/edit-before-accept,
the §8.10 tracked-change markup + DiffView toggle, the §8.11
manual-edit flushes with the stale-change mechanic, the §8.12
range and paragraph sub-threads, the §8.13 flag affordance, and
the §8.14 discuss-mode buffer.

Backend: bot.py grew per-RFC-repo write ops (cut_branch_from_main,
commit_accepted_change with the structured original/proposed/reason
body and Change-Id + Source-Message-Id + On-behalf-of trailers,
commit_manual_flush, ensure_rfc_repo_seed). cache.py grew
refresh_rfc_repo and the webhook dispatches on repository.full_name.
providers.py and chat.py port the §18 carryovers — multi-provider
LLM abstraction and SSE-streaming chat against the §5 threads /
thread_messages / changes schema. api_branches.py mounts the §17
branches/<branch>/* and threads/<thread_id>/* routes with the §6
/ §11 permission checks inline.

Frontend: RFCView.jsx rebuilt as the §8 surface; Editor.jsx,
ChatPanel.jsx, ChangePanel.jsx, PromptBar.jsx, SelectionTooltip.jsx,
DiffView.jsx, ModelPicker.jsx, modelStyles.js lifted from the
prototype and adapted to the canonical schema.

Covered by `backend/tests/test_rfc_view_vertical.py` — eleven new
integration tests against an extended FakeGitea (PUT contents,
POST orgs/{org}/repos, seed_rfc_repo): main-view read,
promote-to-branch, accept (with and without edit-before-accept),
decline, manual flush + system message, flag creation, visibility
flip, anonymous read-but-no-contribute, stale-change refusal, and
the chat-streaming path with a fake provider injected. The 5
Slice 1 tests continue to pass alongside.

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

backend/app/main.py

@@ -14,7 +14,7 @@ from fastapi import APIRouter, FastAPI, HTTPException, Request
 from fastapi.responses import RedirectResponse
 from starlette.middleware.sessions import SessionMiddleware
 
-from . import api as api_routes, auth, cache, db, webhooks
+from . import api as api_routes, auth, cache, db, providers as providers_mod, webhooks
 from .bot import Bot
 from .config import load_config
 from .gitea import Gitea
@@ -32,13 +32,24 @@ async def lifespan(app: FastAPI):
     bot = Bot(gitea)
     reconciler = cache.Reconciler(config, gitea)
 
+    # §18 carryover: the multi-provider LLM abstraction. Provider
+    # construction can fail (missing key, wrong env value) — if it does,
+    # the rest of the app still serves; chat endpoints surface a clear
+    # 503 instead of crashing the process.
+    try:
+        providers = providers_mod.load_from_config(config)
+    except Exception:
+        log.exception("provider construction failed; chat will be disabled")
+        providers = {}
+
     app.state.config = config
     app.state.gitea = gitea
     app.state.bot = bot
     app.state.reconciler = reconciler
+    app.state.providers = providers
 
     app.include_router(_oauth_router(config))
-    app.include_router(api_routes.make_router(config, gitea, bot))
+    app.include_router(api_routes.make_router(config, gitea, bot, providers))
     app.include_router(webhooks.make_router(config, gitea))
 
     reconciler.start()