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/workspace.sql

@@ -13,8 +13,8 @@ SELECT * FROM workspace
 WHERE slug = $1;
 
 -- name: CreateWorkspace :one
-INSERT INTO workspace (name, slug, description, context)
-VALUES ($1, $2, $3, $4)
+INSERT INTO workspace (name, slug, description, context, issue_prefix)
+VALUES ($1, $2, $3, $4, $5)
 RETURNING *;
 
 -- name: UpdateWorkspace :one
@@ -24,9 +24,15 @@ UPDATE workspace SET
     context = COALESCE(sqlc.narg('context'), context),
     settings = COALESCE(sqlc.narg('settings'), settings),
     repos = COALESCE(sqlc.narg('repos'), repos),
+    issue_prefix = COALESCE(sqlc.narg('issue_prefix'), issue_prefix),
     updated_at = now()
 WHERE id = $1
 RETURNING *;
 
+-- name: IncrementIssueCounter :one
+UPDATE workspace SET issue_counter = issue_counter + 1
+WHERE id = $1
+RETURNING issue_counter;
+
 -- name: DeleteWorkspace :exec
 DELETE FROM workspace WHERE id = $1;