feat: replace Directus with custom Node.js GraphQL backend

Removes Directus 11 and replaces it with a lean, purpose-built backend:
- packages/backend/: Fastify v5 + GraphQL Yoga v5 + Pothos (code-first)
  with Drizzle ORM, Redis sessions (session_token cookie), argon2 auth,
  Nodemailer, fluent-ffmpeg, and full gamification system ported from bundle
- Frontend: @directus/sdk replaced by graphql-request v7; services.ts fully
  rewritten with identical signatures; directus.ts now re-exports from api.ts
- Cookie renamed directus_session_token → session_token
- Dev proxy target updated 8055 → 4000
- compose.yml: Directus service removed, backend service added (port 4000)
- Dockerfile.backend: new multi-stage image with ffmpeg
- Dockerfile: bundle build step and ffmpeg removed from frontend image
- data-migration.ts: one-time script to migrate all Directus/sexy_ tables

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/backend/src/db/schema/files.ts

@@ -0,0 +1,27 @@
+import {
+  pgTable,
+  text,
+  timestamp,
+  bigint,
+  integer,
+  index,
+} from "drizzle-orm/pg-core";
+
+export const files = pgTable(
+  "files",
+  {
+    id: text("id").primaryKey().$defaultFn(() => crypto.randomUUID()),
+    title: text("title"),
+    description: text("description"),
+    filename: text("filename").notNull(),
+    mime_type: text("mime_type"),
+    filesize: bigint("filesize", { mode: "number" }),
+    duration: integer("duration"),
+    uploaded_by: text("uploaded_by"),
+    date_created: timestamp("date_created").notNull().defaultNow(),
+  },
+  (t) => [index("files_uploaded_by_idx").on(t.uploaded_by)],
+);
+
+export type File = typeof files.$inferSelect;
+export type NewFile = typeof files.$inferInsert;