feat: fail safe when configuring the optional recency replica

[macdiesel]

Summary

Stacked on top of #118 (the new recency-replica index). Splits the fail-safe behavior into its own PR so the index feature and the resilience change can be reviewed independently.

Base branch: bbeggs/newest-courses-sort-replica (PR #118). I'll retarget this to master once #118 merges. The diff here is only the fail-safe change.

What & why

Configuring the recently-published replica is an optional, additive step. Before this change, if set_replica_index_settings raised AlgoliaException (replica not yet declared, transient Algolia error), the exception propagated out of configure_algolia_index and aborted the entire reindex — taking down configuration of the primary relevance index that every learner's search depends on.

This wraps only the recency-replica step so the exception is logged and skipped, leaving the primary index and its base replica configured. The degraded state is always the safe base (relevance) sort; the recency replica catches up on the next successful run.

• Kept set_replica_index_settings re-raising (a faithful low-level client method); the "this replica is optional" policy lives at the orchestration layer in configure_algolia_index.
• New test test_configure_algolia_index_recency_replica_failure_is_safe forces the replica step to raise and asserts: no propagation, primary + base replica still configured, failure logged.
• ADR 0014 updated with the backend fail-safe decision + consequence.

Test

• 6 targeted tests pass incl. the new safe-fail test; full test_algolia_utils.py green except 3 pre-existing time-sensitive test_get_course_run* failures (fail on master too).
• isort + pylint clean on changed files.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@                          Coverage Diff                          @@
##             bbeggs/newest-courses-sort-replica     #119   +/-   ##
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[macdiesel]

Folded into #118 — the fail-safe commit was cherry-picked onto bbeggs/newest-courses-sort-replica, so the replica feature and its safe configuration now ship as a single PR. Closing this in favor of #118.