feat: add pluggable alarm styles and migrate wecker.py

Introduce a styles/ plugin package:
- styles/base.py: AlarmStyle ABC (injected set_led, update()->bool contract)
- styles/blink.py: BlinkStyle, the existing count-the-blinks puzzle moved
  out of wecker.py's AlarmClock
- styles/simple.py: SimpleStyle, press-once-to-stop (the new default style)
- styles/__init__.py: STYLES registry + get_style() validator + LEGACY_STYLE

wecker.py now resolves the style via the registry and owns only the shared
music/button/cleanup loop; the --style CLI arg defaults to blink so existing
cron entries keep their behaviour. Unknown styles raise before hardware init.

styles/base.py

@@ -0,0 +1,29 @@
+from abc import ABC, abstractmethod
+from typing import Callable
+
+
+class AlarmStyle(ABC):
+    """A pluggable alarm behaviour.
+
+    A style decides *when* the alarm stops and what the LED does while it
+    rings. It never touches hardware directly: ``set_led`` is injected by the
+    runner, so styles stay import-safe in the API process (no GPIO/pygame).
+
+    Lifecycle: construct once with ``set_led``, then call ``update`` every
+    tick. Return ``False`` from ``update`` to stop the alarm.
+    """
+
+    def __init__(self, set_led: Callable[[bool], None]):
+        self.set_led = set_led
+
+    @abstractmethod
+    def update(self, now: float, is_pressed: bool) -> bool:
+        """Advance the style by one tick.
+
+        Args:
+            now: current time (``time.time()`` from the runner).
+            is_pressed: whether the button is currently held down.
+
+        Returns:
+            True to keep ringing, False to stop.
+        """