Add multi-key JWKS rotation for LTI 1.3 signing keys

[donny-wong]

Proposed Changes

(Describe your changes here. Also describe the motivation for your changes: what problem do they solve, or how do they improve the application or codebase? If this pull request fixes an open issue, use a keyword to link this pull request to the issue.)

Summary

MarkUs signs LTI 1.3 client_credentials assertions with a single RSA key and publishes exactly one key at /lti_deployments/public_jwk. Rotating it is therefore a hard swap — the outgoing key leaves the published JWKS the moment it stops signing, so assertions already in flight fail verification.

This adds multi-key support with an overlap window, so keys can be rotated without disrupting grade passback (AGS) or roster sync (NRPS).

What changed

LtiKeyStore (new) manages a directory of timestamped PEMs:

• The newest key is the current signer (overridable via Settings.lti.rotation.current_key).
• public_jwks publishes every key present, so a retired key still verifies until pruned.
• A key is retired when its successor is created. prune! removes keys retired longer ago than the overlap window, and never removes the current signer.
  LtiClient#get_oauth_token and LtiDeploymentsController#public_jwk now both go through the store, so signing and publication share one source of truth.

LtiKeyMaintenanceJob runs daily via resque-scheduler when enabled: rotates only if the current key is past max_age_days, prunes keys past overlap_days. Both are idempotent.

Rake tasks remain for manual use, including compromise response: markus:lti_key, markus:rotate_if_due, markus:prune_keys.

Screenshots of your changes (if applicable)

Type of Change

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Type	Applies?
🚨 Breaking change (fix or feature that would cause existing functionality to change)
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Checklist

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Questions and Comments

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[coveralls]

Coverage Report for CI Build 29976878820

Coverage increased (+0.04%) to 90.45%

Details

• Coverage increased (+0.04%) from the base build.
• Patch coverage: 6 uncovered changes across 1 file (273 of 279 lines covered, 97.85%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
config/initializers/config.rb	6	0	0.0%
Total (12 files)	279	273	97.85%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	51562
Covered Lines:	47654
Line Coverage:	92.42%
Relevant Branches:	2470
Covered Branches:	1218
Branch Coverage:	49.31%
Branches in Coverage %:	Yes
Coverage Strength:	127.22 hits per line

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💛 - Coveralls

[Naragod]

Hey Donny, this is completely new to me, which took me a while to review with the assistance of Claude. I left a few comments based on my understanding of the code and what we are trying to do.