fix(db): share one Prisma client across the server bundles

A production build loads src/lib/db.ts once per server bundle, and the
client was only parked on globalThis outside production. The running
process held three clients: about 28 MB of query-compiler state each on
a heap capped near 520 MB, and three pg pools each sized to the full
per-process connection budget. Park the client on globalThis in every
environment so each copy of the module resolves the same one.

src/lib/__tests__/db.test.ts

@@ -2,7 +2,7 @@
  * Pool configuration stays unit-tested here so connection ceilings and wait
  * timeouts cannot silently fall back to separate library defaults.
  */
-import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 
 import {
   buildSessionOptions,
@@ -129,3 +129,32 @@ describe("buildSessionOptions", () => {
     expect(buildSessionOptions()).toBeUndefined();
   });
 });
+
+describe("process-wide Prisma client", () => {
+  const globalSlot = globalThis as unknown as { prisma?: unknown };
+
+  afterEach(() => {
+    vi.unstubAllEnvs();
+    delete globalSlot.prisma;
+    vi.resetModules();
+  });
+
+  it("shares one client across separate copies of the module in production", async () => {
+    // A production build loads this module once per server bundle; each copy
+    // must resolve the same client rather than constructing its own.
+    vi.stubEnv("NODE_ENV", "production");
+    vi.stubEnv(
+      "DATABASE_URL",
+      "postgresql://healthlog:test@db:5432/healthlog?connection_limit=20",
+    );
+    delete globalSlot.prisma;
+
+    vi.resetModules();
+    const first = (await import("../db")).prisma;
+    vi.resetModules();
+    const second = (await import("../db")).prisma;
+
+    expect(second).toBe(first);
+    expect(globalSlot.prisma).toBe(first);
+  });
+});

src/lib/db.ts

@@ -106,11 +106,16 @@ function createPrismaClient() {
   return new PrismaClient({ adapter });
 }
 
+// One client per process, in production too. A production build gives every
+// server bundle its own copy of this module (the route handlers, the RSC/SSR
+// renderer, and the instrumentation bundle that boots the workers), so a
+// module-scoped client alone meant three of them: three query-compiler states
+// of roughly 28 MB each on a heap that V8 caps near 520 MB in a 1 GiB
+// container, and three pg pools each sized to the whole per-process
+// connection budget above. Parking the client on `globalThis` makes every
+// copy of the module resolve the same one.
 export const prisma = globalForPrisma.prisma ?? createPrismaClient();
-
-if (process.env.NODE_ENV !== "production") {
-  globalForPrisma.prisma = prisma;
-}
+globalForPrisma.prisma = prisma;
 
 // Prisma's `InputJsonValue` requires an explicit index signature that
 // typed application shapes (Zod-validated, hand-written interfaces)