fix: lazy-import tkinter to fix CI on headless Linux

screen_overlay.py imported tkinter at module level, which cascaded
through processor_manager → every test touching the app on CI where
libtk8.6.so is unavailable. Move to TYPE_CHECKING + runtime lazy
import so the overlay module loads cleanly on headless systems.

Also fix test_processor_manager.py to use ProcessorDependencies().

server/src/wled_controller/core/capture/screen_overlay.py

@@ -1,11 +1,15 @@
 """Screen overlay visualization for LED calibration testing."""
 
+from __future__ import annotations
+
 import colorsys
 import logging
 import sys
 import threading
-import tkinter as tk
-from typing import Dict, List, Optional
+from typing import TYPE_CHECKING, Dict, List, Optional
+
+if TYPE_CHECKING:
+    import tkinter as tk
 
 from wled_controller.core.capture.calibration import CalibrationConfig
 from wled_controller.core.capture_engines.base import DisplayInfo
@@ -44,6 +48,8 @@ class OverlayWindow:
 
     def start(self, root: tk.Tk) -> None:
         """Create and show the overlay Toplevel (runs in Tk thread)."""
+        import tkinter as tk  # lazy import — tkinter unavailable in headless CI
+
         self._window = tk.Toplevel(root)
         self._setup_window()
         self._draw_visualization()
@@ -74,6 +80,8 @@ class OverlayWindow:
         win.overrideredirect(True)
         win.attributes("-topmost", True)
 
+        import tkinter as tk
+
         self._canvas = tk.Canvas(
             win,
             width=self.display_info.width,
@@ -270,6 +278,8 @@ class OverlayManager:
 
     def _start_tk_thread(self) -> None:
         def _run():
+            import tkinter as tk  # lazy import — tkinter unavailable in headless CI
+
             try:
                 self._tk_root = tk.Tk()
                 self._tk_root.withdraw()  # invisible root — never shown