server: hand out the Android client this server syncs with (2726)

A self-hoster should not need an account on someone else's forge to get the app
for their own notes. The Fabled-Git instance is private — which is why
`install.sh` already cannot fetch for anyone but the operator — so a release page
is no use as a distribution point. The server holding the notes is something the
person already trusts and already reaches.

It also keeps the pair in step by construction. Client and server negotiate a
sync protocol version before linking, so a server that also serves the client
cannot hand out a phone it is unable to talk to.

**Two files, and both must be present**: `thoughtsync.apk` and a
`thoughtsync-android.json` sidecar carrying `{version_name, version_code, size,
sha256}`. The sidecar exists because an APK keeps its version in a binary AXML
manifest, which Python cannot read and which is not worth putting `aapt` on a
Quart server to reach. CI writes it beside the APK, where the values are already
known — including the digest, computed over the same bytes it uploads, so a
phone can tell a truncated download from a complete one before handing it to the
installer. Not a trust anchor; the signature is that.

**Under DATA_DIR, not baked into the image.** Baking charges ~55 MiB to every
self-hoster including everyone who never touches Android. `/var/thoughtsync` is
already the mounted volume that holds attachments, so a build dropped there
survives container recreation.

**Absence is an ordinary state, not an error.** No APK means the key is absent
from `/api/config` — absent rather than null, so a client testing for it cannot
confuse "this server has no client" with "this server predates the field" — the
web UI hides the card instead of offering a button that 404s, and the metadata
route answers 404. A server whose owner does not use Android is not misconfigured.

**A mismatched pair also counts as no client.** If the sidecar's recorded size
does not match the file on disk, the two did not arrive together; serving one
build while advertising another is worse than serving none, because the phone
would compare versions against a promise the bytes do not keep. That makes the
copy order in docs/android-distribution.md load-bearing, and it is written down
there: APK first, sidecar last.

**The version is public, the bytes are not.** An updater has to be able to ask
"is there something newer?" cheaply and before it has done anything; 55 MiB is
not for anyone who can reach the port. `login_required` already accepts either a
session cookie or a device bearer token, so the browser and a linked phone both
work with no second auth path.

The Android lane now publishes both files to the same rolling `dev` release the
desktop bundles use, reusing `publish-release.sh` — its nullglob asset list was
already built for several jobs in separate workspaces publishing to one release,
which is exactly this. Signed builds only: publishing an unsigned APK would offer
people something they cannot install over what they already have.

Nine tests, DB-free like the rest of the suite — this lane runs no Postgres, so
the advertisement is asserted through `advertisement()` rather than through
`/api/config`, whose other half needs a database. Both routes ARE exercised,
because neither opens a session.

src/thoughtsync/config.py

@@ -33,6 +33,17 @@ class Config:
     def media_root(cls) -> Path:
         return Path(cls.DATA_DIR) / "media"
 
+    @classmethod
+    def client_root(cls) -> Path:
+        """Where the Android APK this server hands out lives.
+
+        Under DATA_DIR rather than baked into the image: the APK is ~55 MiB and an
+        install that never touches Android should not carry it. Being on the same
+        mounted volume as uploads also means an operator drops a build there once
+        and container recreation does not lose it. See client_dist.py.
+        """
+        return Path(cls.DATA_DIR) / "client"
+
     @classmethod
     def secret_key_env(cls) -> str | None:
         """Optional break-glass override for the cookie-signing secret."""