Fix AND filter semantics across local and external event feeds

[dgershman]

Summary

• Local category/tag queries now match nothing when an include filter slug does not resolve in the local taxonomy (e.g. external-only categories like Convention or NJ Region), so AND semantics hold across facets when mixing local and external feeds.
• External feed events are now re-filtered locally for Service Body and Tag selections, mirroring the existing event_type and categories enforcement.

Test plan

• Unit tests for unresolved local category/tag slugs (TaxonomyQueryTest)
• Unit tests for service body and tag external enforcement helpers
• Manual: select Service + external-only category — local Service events should be hidden
• Manual: select Service Body filter — external events from non-Mayo remotes should respect the filter

Closes #323

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