chore: guard against celery disabled

[stevekaplan123]

BLOCKED ON https://app.shortcut.com/sefaria/story/46898/fix-3-failing-pytests#activity-47021

Description

A brief description of the PR

Code Changes

The following changes were made to the files below

Notes

Any additional notes go here

[gitvelocity-reviewer]

📊 Code Quality Score: 1/100

Base 10 × ESF 0.1 (Nano: 3 effective lines, 1 file) = 1

Category	Score	Factors
🔭 Scope	2/20	_ensure_warm() in nonuniqueterm_index.py is the only site touched. No new public API, no new file, no cross-subsystem change.
🏗️ Architecture	1/20	No module boundary changed. A deferred import of CELERY_ENABLED is added inside the function rather than at module level; no new dependency introduced beyond what sefaria.settings already provides.
⚙️ Implementation	2/20	_ensure_warm() gains a single if not CELERY_ENABLED: return branch placed before the existing _get_registry() check, so the function exits without touching _last_rebuild_attempt when Celery is disabled.
⚠️ Risk	3/20	_ensure_warm() is on the cache-read path; returning early when the cache is cold means no rebuild is ever enqueued in non-Celery environments. A misconfigured CELERY_ENABLED=False in production would silently leave the registry cold for all callers.
✅ Quality	1/15	No test accompanies the change. No docstring update reflects the new early-exit condition.
🔒 Perf / Security	1/5	The guard avoids a Celery queue call on every cache-miss in CI and DummyCache environments, eliminating the per-read enqueue described in the existing docstring.

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