test(prisma): guard against duplicate migration numeric prefixes

Two short-lived branches can each grab the next free migration number and
both merge, leaving the tree with two directories sharing a four-digit
prefix. This structural test fails the build the moment a prefix repeats, so
the collision surfaces at PR CI instead of after merge. One historical
collision (0025) predates the guard and is grandfathered — a migration
directory name is the primary key in the applied-migrations ledger, so it
cannot be renamed without orphaning the record on every deployed database.

src/__tests__/migration-number-uniqueness.test.ts

@@ -0,0 +1,69 @@
+/**
+ * Structural guard — every Prisma migration owns a unique numeric prefix.
+ *
+ * Migrations are named `NNNN_description`, and the four-digit prefix is the
+ * ordering key. When two short-lived branches each grab the next free number
+ * and both merge, the tree ends up with two `0268_*` directories. Prisma
+ * applies them in lexical order, but the collision is a silent
+ * merge-time hazard: a reviewer reading `git log` sees one number twice, and a
+ * later renumber has to touch history. This test fails the build the moment a
+ * prefix repeats, so the collision is caught at PR CI rather than after merge.
+ */
+import { readdirSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+import { dirname, resolve } from "node:path";
+
+import { describe, it, expect } from "vitest";
+
+const MIGRATIONS_DIR = resolve(
+  dirname(fileURLToPath(import.meta.url)),
+  "../../prisma/migrations",
+);
+
+const NUMBERED_MIGRATION = /^(\d+)_/;
+
+/**
+ * One grandfathered collision that predates this guard. `0025_refresh_tokens`
+ * and `0025_user_locale_drift_fix` are both long applied on every deployed
+ * database — a migration directory name is the primary key in the
+ * `_prisma_migrations` ledger, so renaming either would orphan the applied
+ * record and force a re-run on every tenant. It is frozen as history; the
+ * guard grandfathers exactly this prefix and refuses every NEW one.
+ */
+const GRANDFATHERED_PREFIXES = new Set(["0025"]);
+
+describe("prisma migrations — numeric prefixes are unique", () => {
+  it("has no two migration directories sharing a numeric prefix", () => {
+    const prefixes = new Map<string, string[]>();
+    for (const entry of readdirSync(MIGRATIONS_DIR, { withFileTypes: true })) {
+      if (!entry.isDirectory()) continue;
+      const match = NUMBERED_MIGRATION.exec(entry.name);
+      if (!match) continue;
+      const prefix = match[1];
+      const bucket = prefixes.get(prefix) ?? [];
+      bucket.push(entry.name);
+      prefixes.set(prefix, bucket);
+    }
+
+    const collisions = [...prefixes.entries()].filter(
+      ([prefix, names]) =>
+        names.length > 1 && !GRANDFATHERED_PREFIXES.has(prefix),
+    );
+
+    expect(
+      collisions,
+      collisions.length > 0
+        ? `Duplicate migration prefixes:\n${collisions
+            .map(([prefix, names]) => `  ${prefix}: ${names.join(", ")}`)
+            .join("\n")}`
+        : undefined,
+    ).toEqual([]);
+  });
+
+  it("finds the migration set (guards against a broken directory path)", () => {
+    const numbered = readdirSync(MIGRATIONS_DIR, {
+      withFileTypes: true,
+    }).filter((e) => e.isDirectory() && NUMBERED_MIGRATION.test(e.name));
+    expect(numbered.length).toBeGreaterThan(0);
+  });
+});