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/223015c/assets/klausur-pattern.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="80" height="80">
+  <rect width="80" height="80" fill="#fce4ec"/>
+  <path d="M-20,20 l40,-40 M0,80 l80,-80 M60,100 l40,-40" stroke="#ffffff" stroke-width="40"/>
+</svg>