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

Doxyfile

@@ -0,0 +1,21 @@
+PROJECT_NAME           = "ptt-fhss"
+PROJECT_BRIEF          = "Bare-metal PTT firmware for nRF52840 with FHSS"
+PROJECT_NUMBER         =
+OUTPUT_DIRECTORY       = docs
+INPUT                  = include src
+FILE_PATTERNS          = *.h *.c
+RECURSIVE              = NO
+EXTRACT_ALL            = YES
+EXTRACT_STATIC         = YES
+QUIET                  = YES
+WARNINGS               = YES
+WARN_IF_UNDOCUMENTED   = YES
+WARN_NO_PARAMDOC       = YES
+GENERATE_HTML          = YES
+HTML_OUTPUT            = html
+HTML_TIMESTAMP         = NO
+GENERATE_LATEX         = NO
+HAVE_DOT               = NO
+OPTIMIZE_OUTPUT_FOR_C  = YES
+JAVADOC_AUTOBRIEF      = YES
+PREDEFINED             = NRF52840_XXAA