feat: disciple link

migrations/0045_add_users_disciple_id.up.sql

@@ -0,0 +1,19 @@
+-- Direct-disciple linkage. Śrīla Prabhupāda's own initiated disciples have
+-- started joining the platform; an admin links their account to the matching
+-- row in the Mongo `disciples` collection so the site can mark them
+-- authentically (a badge on their profile, a highlight in /disciples).
+--
+-- disciple_id holds the Mongo ObjectID hex (24 chars). It's a cross-store
+-- reference, so no foreign key is possible — the admin endpoint validates the
+-- id exists in Mongo before writing, and a dangling id degrades gracefully
+-- (the name simply doesn't hydrate).
+--
+-- The partial UNIQUE index enforces the one-to-one rule: a given disciple can
+-- be claimed by at most one account, so two users can't both present as the
+-- same disciple. NULLs are unconstrained, which leaves the overwhelming
+-- majority of users unaffected and keeps the index tiny.
+ALTER TABLE users
+    ADD COLUMN IF NOT EXISTS disciple_id TEXT;
+
+CREATE UNIQUE INDEX IF NOT EXISTS users_disciple_id_key
+    ON users (disciple_id) WHERE disciple_id IS NOT NULL;