fix: create prosody_unique_index on upgraded databases

Prosody 13's mod_storage_sql upgrade path only warns when the index is
missing (it is created only on fresh tables). On DBs upgraded from 0.11.6
the table pre-existed, so the index was never created and has_upsert_index
stayed false, disabling ON CONFLICT upserts and degrading write performance.

Add an idempotent CREATE UNIQUE INDEX IF NOT EXISTS to both the migration
and new-deployment scripts.

Part-of: <http://gitlab.vnc.biz/uxf/vnctalk-prosody/-/merge_requests/4>

db-customization/prosody-13-migration-once.sql

@@ -35,6 +35,20 @@ begin
   end if;
 end $$;
 
+-- ---------------------------------------------------------------------------
+-- 0. Create the unique index that Prosody 13 expects on the `prosody` table.
+--
+-- On a fresh DB, mod_storage_sql creates `prosody_unique_index` together with
+-- the table. On a DB upgraded from 0.11.6 the table already existed, so
+-- Prosody's upgrade path only *warns* ("Index prosody_unique_index does not
+-- exist") and sets has_upsert_index=false — disabling ON CONFLICT upserts and
+-- falling back to slower SELECT-then-INSERT/UPDATE. Create it here so the
+-- index exists after migration and the warning is gone on next startup.
+-- Idempotent: safe to re-run.
+-- ---------------------------------------------------------------------------
+create unique index if not exists prosody_unique_index
+  on prosody ("host", "user", "store", "key");
+
 -- ---------------------------------------------------------------------------
 -- 1. Drop obsolete objects that referenced the old `_affiliations` / `_occupants` rows
 -- ---------------------------------------------------------------------------

db-customization/prosody-13-new-deployment.sql

@@ -44,6 +44,16 @@ end $$;
 
 create extension if not exists pgcrypto;
 
+-- ---------------------------------------------------------------------------
+-- prosody_unique_index: the unique index mod_storage_sql creates on the
+-- `prosody` table (host, user, store, key). On a truly fresh DB Prosody
+-- creates this itself via create_table(); re-creating it here with IF NOT
+-- EXISTS is a harmless no-op in that case and fixes deployments where the
+-- table was created by an older Prosody version without the unique index.
+-- ---------------------------------------------------------------------------
+create unique index if not exists prosody_unique_index
+  on prosody ("host", "user", "store", "key");
+
 -- =============================================================================
 -- TABLES
 -- =============================================================================