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.prod.example

@@ -53,6 +53,12 @@ ENABLE_ACCOUNT_REGISTRATION=false
 # Register redirect URIs:
 #   https://chatbackend.aimloperations.com/api/auth/oauth/google/callback/
 #   https://chatbackend.aimloperations.com/api/auth/oauth/microsoft/callback/
+# Same client id/secret pair covers Drive linking (#47); no extra IdP app
+# registration needed, but do register the Drive/Graph API + consent screen
+# scopes below in each console (incremental scopes requested at intent time):
+#   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=
@@ -60,6 +66,13 @@ MICROSOFT_OAUTH_CLIENT_SECRET=
 MICROSOFT_OAUTH_TENANT=common
 OAUTH_CALLBACK_BASE_URL=https://chatbackend.aimloperations.com
 
+# Drive / RAG sync (#47-#53) — gated by SubscriptionPlan.allows_rag.
+# Register provider push notifications (Google Drive `watch`, Microsoft
+# Graph subscriptions) against:
+#   https://chatbackend.aimloperations.com/api/drive/webhooks/google/
+#   https://chatbackend.aimloperations.com/api/drive/webhooks/microsoft/
+# Scheduled sync (cron / server-infra job): `python manage.py sync_drive_connections`
+
 # Stripe / finance
 STRIPE_SECRET_KEY=replace-with-stripe-secret-key
 STRIPE_PUBLISHABLE_KEY=replace-with-stripe-publishable-key