Adds module-level 5s debounce (_is_debounced) with thread-safe dict +
GC. Inserts debounce guard in nfc_tap immediately after uid validation.
Adds TestTapEndpointErrors (6 tests): unknown_card, clock_disabled,
no_location_configured, kiosk_disabled, invalid_uid, debounce.
Adds setUp() to both tap test classes to clear _recent_taps between
tests, preventing cross-test debounce bleed. 29/29 pass.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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