fix(handler): set issue_prefix when auto-creating workspace on first login

ensureUserWorkspace in auth.go omitted the IssuePrefix field when
creating a default workspace during first login. This caused the
workspace's issue_prefix to be empty, producing identifiers like "-16"
instead of "JIA-16".

- Pass generateIssuePrefix(wsName) when creating the default workspace
- Add fallback in getIssuePrefix to regenerate from workspace name if
  the stored prefix is empty
- Add migration 024 to backfill empty prefixes on existing workspaces

server/internal/handler/auth.go

@@ -133,10 +133,12 @@ func (h *Handler) ensureUserWorkspace(ctx context.Context, user db.User) error {
 		return nil
 	}
 
+	wsName := defaultWorkspaceName(user)
 	workspace, err := qtx.CreateWorkspace(ctx, db.CreateWorkspaceParams{
-		Name:        defaultWorkspaceName(user),
+		Name:        wsName,
 		Slug:        defaultWorkspaceSlug(user),
 		Description: pgtype.Text{},
+		IssuePrefix: generateIssuePrefix(wsName),
 	})
 	if err != nil {
 		if isUniqueViolation(err) {

server/internal/handler/handler.go

@@ -250,12 +250,17 @@ func splitIdentifier(id string) *identifierParts {
 }
 
 // getIssuePrefix fetches the issue_prefix for a workspace.
+// Falls back to generating a prefix from the workspace name if the stored
+// prefix is empty (e.g. workspaces created before the prefix was introduced).
 func (h *Handler) getIssuePrefix(ctx context.Context, workspaceID pgtype.UUID) string {
 	ws, err := h.Queries.GetWorkspace(ctx, workspaceID)
 	if err != nil {
 		return ""
 	}
-	return ws.IssuePrefix
+	if ws.IssuePrefix != "" {
+		return ws.IssuePrefix
+	}
+	return generateIssuePrefix(ws.Name)
 }
 
 func (h *Handler) loadAgentForUser(w http.ResponseWriter, r *http.Request, agentID string) (db.Agent, bool) {

server/migrations/024_backfill_empty_issue_prefix.down.sql

@@ -0,0 +1 @@
+-- No-op: we cannot reliably determine which workspaces previously had empty prefixes.

server/migrations/024_backfill_empty_issue_prefix.up.sql

@@ -0,0 +1,8 @@
+-- Backfill workspaces that have an empty issue_prefix (e.g. auto-created
+-- during first login before the prefix was wired up in ensureUserWorkspace).
+UPDATE workspace SET issue_prefix = UPPER(
+    LEFT(REGEXP_REPLACE(name, '[^a-zA-Z]', '', 'g'), 3)
+) WHERE issue_prefix = '';
+
+-- Fallback for workspaces whose name has no alphabetic characters.
+UPDATE workspace SET issue_prefix = 'WS' WHERE issue_prefix = '';