Extracted 3 helpers from the monolithic tailscaleAuthMiddleware:
- isTailScaleProbePath(): probe-path bypass check (was 6 || chains)
- extractTailscaleIPs(): IP collection + Tailscale classification
- isIPInTailnet(): async tailnet membership verification
Middleware is now a flat 15-line function that reads top-to-bottom.
Probe paths extracted to a Set for O(1) lookup.
Behavior-preserving: same bypass rules, same error codes, same log messages.
ESLint complexity 24→7, max-depth 6→3. 1539/1539 tests pass.