Fix 9 security vulnerabilities found in audit

[etr]

Summary

Fixes 9 security vulnerabilities identified in a security audit, ranging from HIGH to LOW severity:

• [HIGH] Path traversal in file uploads — When generate_random_filename_on_upload is disabled, a crafted filename field (e.g. ../escape) could write files outside the upload directory. Added sanitize_upload_filename() that extracts the basename and rejects ., .., and empty names.
• [MEDIUM] TOCTOU race in file_response — Replaced stat()-then-open() with open()-then-fstat() to eliminate the race window. Added O_NOFOLLOW on non-Windows to prevent symlink following.
• [MEDIUM] FD leak on lseek failure — file_response::get_raw_response() leaked the fd when lseek() returned -1. Now closes fd before returning nullptr.
• [MEDIUM] NULL pointer dereference — MHD_get_connection_info() can return nullptr; added null check before dereferencing conninfo->connect_fd for TCP_NODELAY.
• [MEDIUM] Uninitialized _file_size — file_info::_file_size was uninitialized in the default constructor. Now zero-initialized.
• [MEDIUM] Auth skip path bypass — should_skip_auth() now normalizes paths (resolves .. and . segments) before comparing, preventing bypass via /public/../protected.
• [LOW] free() vs MHD_free() — Digest auth username was freed with free() instead of MHD_free(). Fixed to use the correct deallocator.
• [LOW] Unchecked write error — File upload writes were not checked for errors. Added !good() check after write(), returning MHD_NO on failure.
• [LOW] FD leak on zero-size file — Zero-size files took a code path that didn't close the fd. Now closes fd before creating an empty buffer response.

Test plan

• All 14 existing tests pass (make check)
• New test: path traversal filename (../escape) is rejected, no file created outside upload dir
• New test: path-like filename (some/path/myfile.txt) is sanitized to basename only
• New test: /public/../protected requires auth (not bypassed via .. traversal)

[codecov]

Codecov Report

❌ Patch coverage is 34.88372% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.93%. Comparing base (755ecc1) to head (28fa48f).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/webserver.cpp	37.03%	3 Missing and 14 partials ⚠️
src/file_response.cpp	12.50%	2 Missing and 5 partials ⚠️
src/http_utils.cpp	42.85%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@ Coverage Diff @@## master #366 +/- ##
==========================================
- Coverage 68.70% 67.93% -0.78% 
==========================================
Files 28 28 Lines 1601 1634 +33 Branches 651 672 +21 ==========================================
+ Hits 1100 1110 +10 - Misses 58 64 +6 - Partials 443 460 +17 

Files with missing lines	Coverage Δ
src/http_request.cpp	60.41% <100.00%> (ø)
src/httpserver/file_info.hpp	100.00% <ø> (ø)
src/httpserver/http_utils.hpp	75.00% <ø> (ø)
src/http_utils.cpp	66.03% <42.85%> (-0.80%)	⬇️
src/file_response.cpp	35.71% <12.50%> (-18.84%)	⬇️
src/webserver.cpp	56.33% <37.03%> (-0.98%)	⬇️

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