feat: add organization sessions mode for Devin self-serve usage tracking

Add `DEVIN_MODE` configuration supporting `enterprise` (default) and `organization_sessions` modes. Organization sessions mode uses `/v3/organizations/{org_id}/sessions` API with `ViewOrgSessions` permission, suitable for non-Enterprise organizations. Sums session ACUs consumed within configurable rolling window (1-365 days via `DEVIN_SESSION_WINDOW_DAYS`). Mode does not show quota, percentage, credit, or reset fields as these are unav

.env.example

@@ -6,8 +6,10 @@ CODEX_TIMEOUT=8
 DEVIN_ENABLED=true
 DEVIN_API_KEY=
 DEVIN_API_URL=https://api.devin.ai
+DEVIN_MODE=enterprise
 DEVIN_ORG_ID=
 DEVIN_USER_ID=
+DEVIN_SESSION_WINDOW_DAYS=30
 DEVIN_TIMEOUT=8
 PROVIDER_CACHE_TTL=8
 REFRESH_INTERVAL=10