Remove dead code and use the event constants consistently

- drop unused BAR import, _VALID_MODES, _is_optional(),
  OPTIONAL_IMPORTS and is_loopback()
- replace hand-written event name strings with EVENT_INSERTED,
  EVENT_REMOVED and EVENT_INVALID, so a typo cannot silently
  disable an event path
- drop the now-moot pytest/coverage entries from .gitignore

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/rdm_card_reader/demo.py

@@ -7,6 +7,9 @@ import time
 
 from .card import (
     DEFAULT_HEARTBEAT_INTERVAL,
+    EVENT_INVALID,
+    EVENT_INSERTED,
+    EVENT_REMOVED,
     CardData,
     CardEvent,
     format_event,
@@ -37,12 +40,12 @@ def run_demo(
 
     for _ in range(max(1, count)):
         card = rng.choice(DEMO_CARDS)
-        event_name = "card_inserted" if card.is_valid else "invalid_card"
+        event_name = EVENT_INSERTED if card.is_valid else EVENT_INVALID
         _emit(CardEvent.from_card(event_name, card, "demo"), as_json)
 
         time.sleep(max(0.1, heartbeat))
         if card.is_valid:
-            _emit(CardEvent.from_card("card_removed", card, "demo"), as_json)
+            _emit(CardEvent.from_card(EVENT_REMOVED, card, "demo"), as_json)
         time.sleep(0.3)
 
     print("\nDemo finished.", flush=True)