restructure: rename termin to kapitel, flatten folder structure

- rename slide files: YYYY-MM-DD-termin-N-topic.md → NN-topic.md
- flatten folders: courses/X/slides/ → slides/X/
- replace "Termin" with "Kapitel" in all content
- add klausur extraction script (make klausur)
- update Makefile, generate-index.sh, dev-server.sh
- add README.md with full documentation

slides/223015b/assets/klausur-pattern.svg

@@ -0,0 +1,9 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="80" height="80">
+  <defs>
+    <pattern id="diagonalStripes" patternUnits="userSpaceOnUse" width="80" height="80" patternTransform="rotate(135)">
+      <rect width="40" height="80" fill="#e3f2fd"/>
+      <rect x="40" width="40" height="80" fill="#ffffff"/>
+    </pattern>
+  </defs>
+  <rect width="100%" height="100%" fill="url(#diagonalStripes)"/>
+</svg>