perf(test): build baseline DB once via vitest globalSetup (#829)

Each test file's setupTestDb() previously re-ran the full
DatabaseService init path: initSchema (~30 CREATE TABLE IF NOT
EXISTS), 14 idempotent migrate*() methods, a bcrypt hash, and the
admin / settings seed inserts. With 82 files this was a meaningful
slice of the per-fork cold-start cost.

Move the build into a vitest globalSetup that runs once before any
worker boots. The baseline DB lands at a fixed temp path; each
worker's setupTestDb copies it into the per-file data dir, opens the
copy via DatabaseService.getInstance() (re-running the same
idempotent init as a no-op pass), then UPDATEs the seeded local
node's compose_dir to match the per-file COMPOSE_DIR (the baseline
recorded /app/compose because COMPOSE_DIR was unset when the seed
fired in initSchema; without realigning, file-routes tests 400 on
path traversal).

TEST_JWT_SECRET moves from a per-file randomBytes assignment to a
fixed constant in a new testConstants module so the value the
baseline seeds matches the value test files import for direct token
signing. setupTestDb re-exports it for back-compat with the existing
import sites.

A baseline-less measurement on the same machine flakes 30 of 82
files at the no-cap baseline; with this baseline copy, the same
tree drops to 0-3 failures (the residual environmental Windows
flakes) and ~47-52 s wall time.

backend/vitest.config.ts

@@ -6,6 +6,9 @@ export default defineConfig({
     // Only run TypeScript sources - exclude the compiled dist/ output.
     include: ['src/__tests__/**/*.test.ts'],
     exclude: ['dist/**', 'node_modules/**'],
+    // Build the baseline DB (schema + migrations + admin seed) once; each
+    // test file's setupTestDb copies it instead of re-running migrations.
+    globalSetup: ['./src/__tests__/helpers/vitestGlobalSetup.ts'],
     // Each test file gets its own worker so singletons are fresh between files.
     pool: 'forks',
     // Cap concurrency: each worker dynamic-imports the full Express stack