fix(migrations): include client_id in platform account unique index

Why: prior migration created the workspace identity unique index without
client_id, so accounts on the same workspace+platform+uid couldn't coexist
across desktop clients. Also drop the legacy uk_platform_accounts_identity_active
index so re-runs are idempotent.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/migrations/20260421100010_add_desktop_fields_to_platform_accounts.up.sql

@@ -53,7 +53,7 @@ CREATE UNIQUE INDEX IF NOT EXISTS uniq_platform_accounts_desktop_id
     ON platform_accounts (desktop_id);
 
 CREATE UNIQUE INDEX IF NOT EXISTS uniq_platform_accounts_workspace_identity_active
-    ON platform_accounts (workspace_id, platform_id, platform_uid)
+    ON platform_accounts (workspace_id, client_id, platform_id, platform_uid)
     WHERE deleted_at IS NULL;
 
 CREATE INDEX IF NOT EXISTS idx_platform_accounts_client_id

server/migrations/20260506120000_scope_platform_accounts_to_desktop_client.down.sql

@@ -1,5 +1,6 @@
+DROP INDEX IF EXISTS uk_platform_accounts_identity_active;
 DROP INDEX IF EXISTS uniq_platform_accounts_workspace_identity_active;
 
 CREATE UNIQUE INDEX IF NOT EXISTS uniq_platform_accounts_workspace_identity_active
-    ON platform_accounts (workspace_id, platform_id, platform_uid)
+    ON platform_accounts (workspace_id, client_id, platform_id, platform_uid)
     WHERE deleted_at IS NULL;

server/migrations/20260506120000_scope_platform_accounts_to_desktop_client.up.sql

@@ -1,3 +1,4 @@
+DROP INDEX IF EXISTS uk_platform_accounts_identity_active;
 DROP INDEX IF EXISTS uniq_platform_accounts_workspace_identity_active;
 
 CREATE UNIQUE INDEX IF NOT EXISTS uniq_platform_accounts_workspace_identity_active