Take the link from wg-app-link instead of keeping a second copy

The five modules underneath this backend that were never about AI
sessions -- the pinned CA and leaf, QR enrollment and the bearer token,
wg0 binding and the certificate's SANs, owner-only files, and the RON
house rules -- were written twice, once here and once in dev-updater,
and had drifted. They now come from the submodule, as a path dependency
so both projects stay locked to one commit.

What stayed is what makes this project itself: the routes, the drivers,
the config schema, and the auth middleware, which is generic over this
server's state. Sharing a transport is worth doing; sharing an API would
mean inventing a vocabulary neither project wants.

Four dependencies go with the code -- rcgen, qrcode, subtle and if-addrs
are no longer named here at all -- and the three that remain are now
described by what still uses them rather than by what used to.

Verified by running it, not only by building: a fresh server generates
its CA, prints an `aiapp://enroll` QR with the scheme now passed as a
parameter, covers 127.0.0.1, 10.0.2.2 and wg0's 10.66.0.1 in the leaf,
answers an enrolled token and returns 401 without one, and writes
config.ron in the house rules with every file owner-only. 36 tests pass,
clippy is silent, rustfmt is clean.

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "wg-app-link"]
+	path = wg-app-link
+	url = git@git.arirex.me:iris/wg-app-link.git