feat(issues): add human-readable issue identifiers (e.g. JIA-1)

Add per-workspace auto-incrementing issue numbers with a configurable
prefix, producing identifiers like "JIA-1" instead of truncated UUIDs.

Database:
- Add issue_prefix and issue_counter to workspace table
- Add number column to issue table with UNIQUE(workspace_id, number)
- Backfill existing issues with sequential numbers

Backend:
- Issue creation atomically increments counter in a transaction
- API responses include number and identifier fields
- Support issue lookup by identifier format (KEY-N)
- Workspace prefix auto-generated from name, customizable via API

Frontend:
- Display identifier in list rows and issue detail breadcrumb
- Add issue_prefix to Workspace type, number/identifier to Issue type

server/pkg/db/queries/issue.sql

@@ -15,11 +15,15 @@ WHERE id = $1;
 INSERT INTO issue (
     workspace_id, title, description, status, priority,
     assignee_type, assignee_id, creator_type, creator_id,
-    parent_issue_id, position, due_date
+    parent_issue_id, position, due_date, number
 ) VALUES (
-    $1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12
+    $1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13
 ) RETURNING *;
 
+-- name: GetIssueByNumber :one
+SELECT * FROM issue
+WHERE workspace_id = $1 AND number = $2;
+
 -- name: UpdateIssue :one
 UPDATE issue SET
     title = COALESCE(sqlc.narg('title'), title),