chat: simplex_name partial indexes are UNIQUE

A simplex name is a stable, per-user identity (one name → one contact
or group). Without a unique constraint, a later writer that populates
the column twice for the same name would silently produce two matching
rows, and getContactBySimplexName/getGroupInfoBySimplexName would
return whichever the planner picks first.

Promote the partial indexes added in M20260603 to UNIQUE before any
caller wires the writes. Predicate (WHERE simplex_name IS NOT NULL)
already scopes the constraint to rows that opted in.

src/Simplex/Chat/Store/Postgres/Migrations/M20260603_simplex_name.hs

@@ -13,11 +13,11 @@ ALTER TABLE contacts ADD COLUMN simplex_name TEXT;
 ALTER TABLE groups ADD COLUMN simplex_name TEXT;
 ALTER TABLE connections ADD COLUMN simplex_name TEXT;
 
-CREATE INDEX idx_contacts_simplex_name
+CREATE UNIQUE INDEX idx_contacts_simplex_name
   ON contacts(user_id, simplex_name)
   WHERE simplex_name IS NOT NULL;
 
-CREATE INDEX idx_groups_simplex_name
+CREATE UNIQUE INDEX idx_groups_simplex_name
   ON groups(user_id, simplex_name)
   WHERE simplex_name IS NOT NULL;
 |]

src/Simplex/Chat/Store/SQLite/Migrations/M20260603_simplex_name.hs

@@ -12,11 +12,11 @@ ALTER TABLE contacts ADD COLUMN simplex_name TEXT;
 ALTER TABLE groups ADD COLUMN simplex_name TEXT;
 ALTER TABLE connections ADD COLUMN simplex_name TEXT;
 
-CREATE INDEX idx_contacts_simplex_name
+CREATE UNIQUE INDEX idx_contacts_simplex_name
   ON contacts(user_id, simplex_name)
   WHERE simplex_name IS NOT NULL;
 
-CREATE INDEX idx_groups_simplex_name
+CREATE UNIQUE INDEX idx_groups_simplex_name
   ON groups(user_id, simplex_name)
   WHERE simplex_name IS NOT NULL;
 |]

src/Simplex/Chat/Store/SQLite/Migrations/chat_schema.sql

@@ -1310,13 +1310,13 @@ ON groups(
   relay_request_group_link
 )
 WHERE relay_request_group_link IS NOT NULL;
-CREATE INDEX idx_contacts_simplex_name
+CREATE UNIQUE INDEX idx_contacts_simplex_name
 ON contacts(
   user_id,
   simplex_name
 )
 WHERE simplex_name IS NOT NULL;
-CREATE INDEX idx_groups_simplex_name
+CREATE UNIQUE INDEX idx_groups_simplex_name
 ON groups(
   user_id,
   simplex_name