Extract Pensive assignment platform integration logic from legacy repository

[cycomachead]

General Info

• Pivotal Tracker Story
• Breaking?

Changes

Extracts and implements Pensieve assignment platform integration, ported from the legacy berkeley-cdss/extensions repo, following the existing Gradescope facade pattern end to end. Adds a new LMS row/facade, DB fields, and course settings UI so extensions can be posted to Pensieve.

• DB entry: Lms row id 3 ("Pensieve") — new PENSIEVE_LMS_ID constant, seed, Lms.PENSIEVE_LMS, and Lms.facade_class mapping. Migration adds enable_pensieve and pensieve_course_url to course_settings.
• Facade: app/facades/pensieve_facade.rb with get_all_assignments, provision_extension, from_user, assignment_url, backed by lib/lmss/pensieve/ (JSON Client plus Assignment/Override value objects), configured via new PENSIEVE_API_TOKEN env var.
• Extension posting: Request#approve now handles PensieveFacade (course URL + student email). Since Pensieve only accepts whole days past the original deadline, the facade converts the approved due date into num_days relative to the synced assignment's due date, and refuses to post if the assignment can't be found or the request grants zero days.
• Course settings UI: new Pensieve card on Course Details (toggle + course URL, mirroring Gradescope). Since Pensieve keys everything by URL, the course URL is stored as external_course_id and assignment URLs as external_assignment_id.
• Docs: new section in docs/integrations.md and .env.example entry for PENSIEVE_API_TOKEN.

Known gaps / open questions for Pensieve

The old system (src/pensieve.py) only supported a single API call (grant-extension) — there was no assignment sync; staff manually entered Pensieve URLs. To fully support sync, we need from Pensieve:

1. An API token (PENSIEVE_API_TOKEN, Bearer auth) — the only credential currently used.
2. An assignment-listing endpoint. None exists today or is documented. get_all_assignments is wired to a proposed, unconfirmed endpoint (GET /api/b2s/v1/external-client/assignments, flagged in lib/lmss/pensieve/client.rb). Until Pensieve confirms a real endpoint/shape, sync is a safe no-op (returns []; the sync job does not disable existing assignments on empty response).

Other semantic limitations to be aware of: extensions are granted in whole days (time-of-day is lost), there's no late/hard due date support (parameter ignored), and extensions can't be read back or revoked via the API (get_assignment_overrides returns []; external_extension_id is only populated if Pensieve happens to return one).

Testing

Added specs for PensieveFacade and Lmss::Pensieve::Client, plus coverage in lms, course_settings, and request specs (approval flow, link creation/update, URL validation). Full suite (749 specs) passes and RuboCop is clean.

Documentation

Added a "Pensieve" section to docs/integrations.md covering setup (API token + course URL) and known limitations (whole-day extensions, no separate late due date).

Checklist

• Name of branch corresponds to story

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