api: add init_transports() for multi-relay onboarding

Closes #8693 and supsersedes #8707 from which the API
and some overall shape of this commit is inspired.

UIs call `init_transports(None)` or `init_transports(qr)`
to initialize a first transport on the fresh profile,
with more transports added in the background later.

docs/schema.sql

@@ -630,9 +630,12 @@ CREATE TABLE broadcast_secrets(
 
 
 -- Candidate chatmail relays for automatic relay management.
+-- Holds the hosts a QR code contributed and the default relays already tried;
+-- the default list itself lives in `autorelay.rs` and is not stored here.
 CREATE TABLE relay_candidates(
     host TEXT PRIMARY KEY NOT NULL,
-    last_tried INTEGER NOT NULL DEFAULT 0 -- Timestamp of the last connection attempt.
+    -- Timestamp of the last connection attempt, 0 if the host was never tried.
+    last_tried INTEGER NOT NULL DEFAULT 0
 ) STRICT;
 
 CREATE TABLE transports (