Extract Pensive assignment platform integration logic from legacy repository

[cycomachead]

General Info

• Pivotal Tracker Story
• Breaking?

Changes

Extracts and implements the Pensive assignment platform integration ported from the legacy extensions repo, per request. This adds Pensive as a third supported LMS:

• New PensiveFacade (registered as LMS id 3) supporting:
  • Assignment sync via a configurable PENSIEVE_ASSIGNMENTS_PATH endpoint (Pensive's legacy API doesn't provide assignment listing, so this must be supplied separately and documented in docs/integrations.md).
  • Posting extensions via Pensive's legacy grant-extension API, using the assignment URL, student email, and a whole-day extension count (rather than absolute due dates like Canvas/Gradescope).
• Lmss::Pensive::Assignment and Lmss::Pensive::Override model classes to normalize Pensive's response payloads into the shared assignment-sync/extension-provisioning pipeline.
• DB migration + seed entry registering Pensive as LMS id 3, plus Lms.PENSIVE_LMS and Course#pensive_id helpers.
• Request#approve updated to special-case Pensive's day-count based extension API while keeping the existing date-based flow for other facades.
• assignment_link_for helper generalized to fall back to assignment.external_url + LMS name for any facade without a hardcoded case (used by Pensive, since assignment URLs are the identifier).
• .env.example and docs/integrations.md updated with required config (PENSIEVE_EMAIL, PENSIEVE_API_TOKEN, PENSIEVE_ASSIGNMENTS_PATH, optional PENSIEVE_API_URL) and the expected assignment-list response contract.

Outstanding items before this can be exercised end-to-end (see conversation for details)

• A Pensive integration account invited to every managed class, with PENSIEVE_EMAIL + an API token from that account.
• A CourseToLms link per course using LMS id 3 and the Pensive class id (extracted from the class URL, e.g. data100_fa26)—currently must be created manually via the existing POST /api/v1/courses/:course_id/lmss API or Rails console; no instructor-facing UI yet.
• Pensive must supply a real assignment-list endpoint matching the documented contract, since the legacy integration only supports posting extensions.
• Network access to api.pensieve.co was not fully available during implementation (403 Forbidden by network policy, then 404s/rate limiting on other paths), so live end-to-end validation against Pensive was not completed.

Testing

• 730 specs passing, RuboCop clean, Brakeman clean.
• Added unit specs for PensiveFacade (assignment sync, extension provisioning, config validation, error handling) using stubbed HTTP requests.
• Added specs for Lms.PENSIVE_LMS/facade_class, assignment_link_for fallback behavior, SyncAllCourseAssignmentsJob with a Pensive course link, and Request#approve dispatching whole-day extensions through PensiveFacade.
• Live smoke testing against the real Pensive API was not possible due to network policy restrictions and missing credentials/endpoint config (see above).

Documentation

Added a "Pensive" section to docs/integrations.md covering required environment variables, course-linking steps, the assignment-sync response contract Pensive must implement, and how extension posting works. .env.example documents the new variables inline.

Checklist

• Name of branch corresponds to story

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