Add tests for bugs page access control and configuration

Shell tests exercising the C plumbing for the bugs page feature.
No Lua or git-bug dependencies required.

Tests cover:
- Bugs tab hidden by default, shown when enable-bugs=1
- Per-repo and global enable-bugs configuration
- 404 response when no bugs-filter is configured
- 403 response when bugs page is not enabled
- Bug ID path does not leak into other nav tab URLs
- cache-bugs-ttl is parsed without error

Signed-off-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Assisted-by: claude-opus-4-6