Tier-gated RAG + Drive document sources (#42) (#54)

## Summary

Implements epic [#42](#42) (children #43–#53) and advances [#11](#11).

- **Entitlement:** `allows_rag` on plans (founders / backer / pro / business; not standard); exposed as `features.rag`
- **Gates:** document REST + WS `PromptType.RAG` use `assert_feature_allowed(..., "rag")`
- **Lifecycle:** dedupe ingest, delete vectors by `document_id`, honor `active`, fix document detail PATCH/DELETE
- **Workspaces:** auto-create default company workspace; fail-closed scoping
- **Drive:** personal + company Google/Microsoft connect (`link_drive` / `link_company_drive`), resource selection, sync, webhooks stubs, `sync_drive_connections` management command
- **Docs/env:** README + `.env*.example` updated

Companion FE: `chat_web_app` branch `feature/rag-epic-42-ui` (#81–#85).

## Test plan

- [x] `SKIP_RAG_INIT=1 uv run python manage.py test` (457 OK)
- [ ] Migrate finance `0004` + chat_backend `0028` on beta
- [ ] Verify Standard user: Documents API 403 + no RAG retrieval
- [ ] Verify Founders/Pro: upload + list + active toggle
- [ ] Connect Google/Microsoft Drive (incremental scopes) and Sync
- [ ] Company manager: `link_company_drive`; non-manager 403
- [ ] Run `manage.py sync_drive_connections`Reviewed-on: #54

.env.example

@@ -32,6 +32,12 @@ ENABLE_ACCOUNT_REGISTRATION=false
 # Redirect URIs (register in each IdP console):
 #   {OAUTH_CALLBACK_BASE_URL}/api/auth/oauth/google/callback/
 #   {OAUTH_CALLBACK_BASE_URL}/api/auth/oauth/microsoft/callback/
+# Same client id/secret pair is reused for Drive linking (#47) — the extra
+# Drive scopes below are requested incrementally via intent=link_drive /
+# intent=link_company_drive, no separate app registration needed:
+#   Google:    openid email profile https://www.googleapis.com/auth/drive.readonly
+#   Microsoft: openid email profile offline_access Files.Read (personal)
+#              openid email profile offline_access Files.Read.All Sites.Read.All (company)
 GOOGLE_OAUTH_CLIENT_ID=
 GOOGLE_OAUTH_CLIENT_SECRET=
 MICROSOFT_OAUTH_CLIENT_ID=
@@ -40,6 +46,22 @@ MICROSOFT_OAUTH_TENANT=common
 # Optional; defaults to request host. Example local: http://127.0.0.1:8001
 OAUTH_CALLBACK_BASE_URL=http://127.0.0.1:8001
 
+# Drive / RAG sync (#47-#53). Requires a subscription plan with allows_rag
+# (Founders, Pro, Business, Backer by default — see finance PLAN_SEED).
+# Start a link:  GET  /api/auth/oauth/google/start/?intent=link_drive             (authenticated)
+#                GET  /api/auth/oauth/google/start/?intent=link_company_drive     (company manager)
+#                GET  /api/auth/oauth/microsoft/start/?intent=link_drive
+#                GET  /api/auth/oauth/microsoft/start/?intent=link_company_drive
+# Manage:        GET    /api/drive/connections/
+#                DELETE /api/drive/connections/<id>/
+#                POST   /api/drive/connections/<id>/resources/   { "resource_ids": [...] }
+#                POST   /api/drive/connections/<id>/sync/
+# Provider push notifications (best-effort; register with each provider's
+# subscription/watch API pointing here, using ?connection_id=<id>):
+#   POST {OAUTH_CALLBACK_BASE_URL}/api/drive/webhooks/google/
+#   POST {OAUTH_CALLBACK_BASE_URL}/api/drive/webhooks/microsoft/
+# Worker sync: `python manage.py sync_drive_connections [--connection-id N]`
+
 # Stripe / finance (optional local — required for checkout + webhooks)
 STRIPE_SECRET_KEY=
 STRIPE_PUBLISHABLE_KEY=