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.

server/Cargo.toml

@@ -8,6 +8,11 @@ name = "ai-server"
 path = "src/main.rs"
 
 [dependencies]
+# The link both this and dev-updater need in order to be reached from a
+# phone: wg binding, the pinned CA, QR enrollment, owner-only files, and
+# the RON house rules. Extracted from the two copies that had drifted --
+# see that repo's README for the evidence and the bug the extraction found.
+wg-app-link = { path = "../wg-app-link/server" }
 axum = { version = "0.8", features = ["json", "multipart"] }
 axum-server = { version = "0.8", features = ["tls-rustls"] }
 tokio = { version = "1", features = ["rt-multi-thread", "macros", "net", "sync", "time", "process", "io-util", "signal"] }
@@ -17,32 +22,19 @@ tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 # The config file's format. Not JSON, because this file is written and read
-# by hand and RON says a sum type as syntax -- the same choice, and the same
-# house rules, as the sibling dev-updater project's config.
+# by hand and RON says a sum type as syntax. The two house rules both
+# projects write it under live in wg-app-link; this is here for the error
+# types the schema's own signatures name.
 ron = "0.12.2"
 clap = { version = "4", features = ["derive"] }
 anyhow = "1"
 thiserror = "2"
-# Token auth: hash for storage, constant-time compare for verification,
-# CSPRNG-backed generation, base64url for the enrollment string.
+# Verifying a downloaded model against HuggingFace's published digest.
 sha2 = "0.11"
-subtle = "2"
+# Naming a session directory, and an attachment inside one.
 rand = "0.10"
+# Decoding the images a phone attaches, and encoding them for a driver.
 base64 = "0.23"
-# Renders the enrollment QR straight to the terminal; no image output needed.
-qrcode = { version = "0.14", default-features = false }
-# The wg0-bound listener needs the interface's address; the stdlib has no
-# getifaddrs. This is the smallest crate that wraps just that.
-if-addrs = "0.15"
-# Generates this server's TLS certificates on first start, replacing a
-# setup script that shelled out to whatever openssl happened to be
-# installed. In process means one place decides the extensions, the file
-# modes, and which addresses the leaf covers. x509-parser so the issuer is
-# read back from the CA actually on disk: reconstructing it from the same
-# parameters would work only as long as nothing ever changed them, and a
-# mismatched issuer name yields a chain that fails to validate rather than
-# anything that looks wrong at generation time.
-rcgen = { version = "0.14", features = ["pem", "x509-parser"] }
 # Outbound HTTPS for the usage endpoint. A small blocking client fits an
 # every-few-minutes poll better than pulling in reqwest's tower stack;
 # rustls-backed like the rest of the TLS here.