Add Doxygen, clang-format, cppcheck, and Gitea CI

Doxygen:
- Doxyfile: minimal config, HTML output to docs/, no LaTeX
- @file/@brief on all source files, full @param/@return on public API
- docs/ added to .gitignore

clang-format (14, Linux brace style, 4-space, column 100):
- .clang-format added
- Applied to entire codebase; this commit is the canonical baseline
- just format rewrites in-place; just format-check is the CI gate

cppcheck (--enable=warning,style,performance,portability):
- Linker-symbol pointer comparisons in startup.c suppressed with
  inline cppcheck-suppress (false positives, not real bugs)
- just lint runs cppcheck; zero warnings required to pass

Dockerfile gains clang-format, cppcheck, doxygen packages so all
tools run inside the existing container -- host stays clean.

Gitea Actions (.gitea/workflows/ci.yml):
- Four parallel jobs: build, format, lint, docs
- All jobs use the same Dockerfile-based image
- Doxygen job fails on any warning line in output

include/power.h

@@ -1,4 +1,16 @@
+/**
+ * @file power.h
+ * @brief Power management: DC/DC regulator, GPIOTE wakeup, SYSTEM_ON sleep.
+ */
 #pragma once
 
+/** @brief Enable the DC/DC converter and configure GPIOTE wakeup on the PTT button. */
 void power_init(void);
-void power_sleep_until_button(void);   /* SYSTEM_ON WFI, woken by GPIOTE event */
+
+/**
+ * @brief Enter SYSTEM_ON low-power sleep and return on the next GPIOTE event.
+ *
+ * Sets TASKS_LOWPWR then executes WFI.  The CPU wakes when the GPIOTE
+ * interrupt fires (button press) and resumes from here.
+ */
+void power_sleep_until_button(void);