chore(deps): upgrade Prisma 6.19 -> 7.8.0 (Rust-free client + mariadb adapter)

prisma / @prisma/client 6.19.2 -> 7.8.0; added @prisma/adapter-mariadb 7.8.0 and
mariadb 3.5.2 (the runtime driver).

Prisma 7 breaking changes handled (verified via Prisma's own tooling — the
official upgrade-guide summary had hallucinated package names, so I confirmed
everything against `prisma validate`/`generate` and the installed type defs):

- The Rust query engine is removed; the runtime connection is now made through a
  driver adapter. src/config/database.ts wires PrismaMariaDb, parsing
  DATABASE_URL into an explicit mariadb pool config so the literal `@` in our
  password (which a naive connection-string parser mishandles) is correct.
- datasource `url` is no longer allowed in schema.prisma -> moved to a new
  prisma.config.ts (schema + datasource.url + migrations.seed). Prisma 7 no
  longer auto-loads .env, so the config does `import "dotenv/config"`.
- The deprecated package.json#prisma seed config was removed (now in the config).
- seed.ts and the two maintenance scripts now use the shared adapter-wired client.

The legacy `prisma-client-js` generator STILL works under v7 (generates to
@prisma/client as before), so NO import-path changes were needed across the 16
@prisma/client import sites — switching to the new prisma-client/output generator
is not required for us. No prisma.$use() middleware exists. Node 24 / TS 5.9
exceed the v7 minimums (20.19 / 5.4).

No migrations were run (your DBs are untouched); removing datasource.url is a
config change, not DDL, so this phase needs no new migration.

Gates: tsc 0 | build 0 | vitest 247/247 against app_test (proves the mariadb
adapter works at runtime, incl. raw SELECT...FOR UPDATE locking and Decimal/VAT)
| eslint 0 errors.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

prisma.config.ts

@@ -0,0 +1,16 @@
+import "dotenv/config";
+import { defineConfig, env } from "prisma/config";
+
+// Prisma 7 moved the datasource connection URL and seed config out of
+// schema.prisma / package.json into this file. `import "dotenv/config"` is
+// required because Prisma 7 no longer auto-loads .env. The runtime database
+// connection itself is made via the driver adapter in src/config/database.ts.
+export default defineConfig({
+  schema: "prisma/schema.prisma",
+  datasource: {
+    url: env("DATABASE_URL"),
+  },
+  migrations: {
+    seed: "tsx prisma/seed.ts",
+  },
+});