fix: admin-room fallback for matrix password reset when Synapse API absent

tuwunel 1.6.x does not implement the Synapse admin REST API.
reset_user_password() now falls back to the Matrix admin room
(#admins:<server>) when PUT /_synapse/admin/v2/users returns 404:

1. Discover admin room ID via #admins:<server> alias
2. Get current messages end-token (pagination anchor)
3. Send 'reset-password @<localpart>:<server>' as @hive admin user
4. Poll for bot response up to 5 x 1s; extract password from message
5. Persist the new password and return it

The function signature changes from Result<()> to Result<String> so the
caller can use the effective password (which may be server-generated on
the admin-room path) for subsequent login calls.

Closes #1267.

hive-c0re/src/bin/hivectl.rs

@@ -495,6 +495,7 @@ async fn matrix_reset_password(name: &str) -> Result<()> {
     )
     .await
     .with_context(|| format!("matrix reset-password {name}"))?;
+    // Password is persisted by reset_user_password (including admin-room path).
     let pw_path = PathBuf::from("/var/lib/hyperhive/matrix-creds").join(format!("{name}-password"));
     println!("matrix: password for @{name}:{server_name} reset");
     println!("password persisted at: {}", pw_path.display());