Remove the TaggableManager monkeypatch by fixing its stale registration #786

Proposed Changes

Problem: The TaggableManager monkeypatch, its global lock and its ValueError swallow work around taggit resolving the stale registered class by name; a polymorphic through is re-registered with an unsuppressed warning.

Fix: Pop the old registration before type() so taggit fires on the new class; remove the patch, lock and re-registers.

Where (line numbers as of a61cd70; they will have drifted): models.py:1905–1961, 1645

Audit: finding E5, severity Medium, effort M, originally batch 5 (#737). Not re-checked after the review: first confirm it against current main (reproduce it, or point to the code) before fixing.

Note: Possibly related to #755 / #756 (stale models left registered in other workers); check whether that fix removes the stale registration this patch works around.

Justification

Medium severity; not a v1.0 blocker. Tracked in #759 under tier 3 (robustness and performance), re-sliced from the pre-1.0 code audit so each fix gets its own reviewable PR.