fix(db): enforce global unique project names across creators

Replace per-creator name uniqueness with a global index among
non-retired projects, and reject cross-user duplicate creates with 409.

Co-authored-by: Cursor <cursoragent@cursor.com>

deploy/postgres/goose/00057_2026-09-20-global-unique-project-name.sql

@@ -0,0 +1,38 @@
+-- +goose Up
+-- 全局项目名唯一：任意活跃项目不得重名（跨创建人）。
+-- 替换 00056 的 per-creator 唯一索引。
+
+-- Deduplicate globally by lower(trim(name)) among non-retired rows (keep lowest id).
+WITH dups AS (
+    SELECT id,
+           ROW_NUMBER() OVER (
+               PARTITION BY lower(trim(name))
+               ORDER BY id ASC
+           ) AS rn
+    FROM projects
+    WHERE length(trim(name)) > 0
+      AND COALESCE(status, 'active') <> 'demo_retired'
+)
+UPDATE projects p
+SET name = p.name || ' (dup-' || p.id || ')',
+    updated_at = now()
+FROM dups d
+WHERE p.id = d.id AND d.rn > 1;
+
+DROP INDEX IF EXISTS uq_projects_creator_name;
+
+CREATE UNIQUE INDEX IF NOT EXISTS uq_projects_name_global
+    ON projects (lower(trim(name)))
+    WHERE length(trim(name)) > 0
+      AND COALESCE(status, 'active') <> 'demo_retired';
+
+COMMENT ON INDEX uq_projects_name_global IS
+    'Global unique project name among non-retired projects (case-insensitive, trimmed).';
+
+-- +goose Down
+DROP INDEX IF EXISTS uq_projects_name_global;
+
+CREATE UNIQUE INDEX IF NOT EXISTS uq_projects_creator_name
+    ON projects (creator_user_id, lower(trim(name)))
+    WHERE creator_user_id IS NOT NULL AND creator_user_id > 0
+      AND length(trim(name)) > 0;