fix: review fixes for webhook and auto mode (#8)

Clear available_* only when the deployed ref equals it, drop the unused
poll source, stop leaking internal errors to CI, use the request-scoped
logger, validate X-Request-ID, re-read jobs once after the timeout,
report refused when no target ref exists, and tidy UI, deploy and docs.

internal/master/store/migrations/0003_webhook.sql

@@ -1,6 +1,6 @@
 ALTER TABLE services ADD COLUMN available_ref    text NOT NULL DEFAULT '';
 ALTER TABLE services ADD COLUMN available_source text NOT NULL DEFAULT ''
-    CHECK (available_source IN ('', 'webhook', 'poll'));
+    CHECK (available_source IN ('', 'webhook'));
 ALTER TABLE services ADD COLUMN available_at     timestamptz;
 
 CREATE INDEX services_by_image_repo ON services (image_repo) WHERE removed_at IS NULL;