fix(#3150): let an SSO login adopt the matrix account it names

An SSO login whose derived localpart already exists silently gets a
random one instead: the candidate must be *available*, an existing
unlinked account is not, and `unique_id_fallbacks` defaults to true.
The operator's own login minted a stray account this way.

Fill the three identity_provider fields that decide which account a
login lands on: `trusted` (upstream's precondition -- self-hosted and
fully controlled -- is structural here, the issuer is always the swarm's
own authelia), `userid_claims` pinned to preferred_username so the
default ladder cannot fall through to an email local part, and
`unique_id_fallbacks = false` so a collision errors instead of
succeeding as the wrong user.

nix/host-modules/hive-matrix.nix

@@ -887,6 +887,30 @@ in
                   # relying on that logs a warning every startup, and a
                   # recurring warning that is expected is one nobody reads.
                   default = true;
+
+                  # Upstream's rule is "only ever set `trusted` for
+                  # identity providers you self-host and fully control",
+                  # and this module cannot point anywhere else: the issuer
+                  # is `swarm.authelia.url`, whose client, secret and user
+                  # database are all ours. It does mean whoever can make
+                  # authelia emit a given name gets that account — for our
+                  # own identity provider that IS the identity.
+                  # Without it, an SSO login cannot adopt an account that
+                  # already exists; it can only ever create a new one.
+                  trusted = true;
+
+                  # One claim instead of upstream's ladder
+                  # (`preferred_username` → `username` → `nickname` →
+                  # `login` → `email`). The tail is the hazard: an email
+                  # local part is a different namespace, so a login can
+                  # land on a name that means someone else here.
+                  userid_claims = [ "preferred_username" ];
+
+                  # The default (`true`) makes a name collision SILENT —
+                  # tuwunel invents a random localpart and the login
+                  # succeeds as the wrong user. `false` errors instead,
+                  # which is the only form of this an operator can act on.
+                  unique_id_fallbacks = false;
                 }
               ];
             };