feat: resolve Phase 1 contract requests and compose every feature

Contracts and core:
- Feature gains auth and stop; FeatureFactory; core composeServer builds the
  context, runs the factories, then the job queue from the union of their jobs
  (R8/R14). startServer uses it; main.ts composes learner, import, studio, AI.
- AppConfig/loadConfig own the layout and settings: courseDirs include
  <dataDir>/courses and <dataDir>/worktrees, studioReposDir, cacheDir,
  importRoots, uploadMaxMb, aiAllowedBaseUrls, transcribeUrl, python.
  resolveConfig derives the same for tests; cachePaths shares the importer's
  cache layout.
- canView: the global author role views every course (R14); the learner,
  studio and AI packages all call it.
- migrate(db, packages): core first, then packages in name order.
- Media: nosniff on every response, a sandbox CSP on svg/html/js; a Range on
  an empty file is 416.
- courseUrl/lessonUrl and the lesson route patterns live in @stc/contracts.

Packages:
- import, studio and AI read AppConfig instead of process.env; the importer
  refuses ids used by studio/repos.
- learner migrations run through core migrate; library covers fall back to
  the importer's cached candidate behind a canView-checked route.
- studio preview renders through the learner LessonView (anchors honoured,
  draft URLs); AI drafts go to the studio draft branch for studio courses (as
  the requesting user) and to <dataDir>/drafts otherwise, and the generator
  page says which.

Web: tsconfig paths for @stc/<feature>/web browser entries; features.ts
imports them by package name; nav.roles hides links by role.

Tests: compose, config, canView, media headers, migrations order, covers,
preview, nav, SPA bundle without server code, and an end-to-end flow on
SQLite and Postgres.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/server/test/pg.ts

@@ -0,0 +1,13 @@
+import { connect } from "@stc/db";
+
+export const PG = process.env.STC_TEST_PG_URL;
+
+/** A fresh Postgres schema (search_path in the URL), dropped by `drop()`. Same isolation as packages/db's forEachDb. */
+export async function pgSchema(base: string): Promise<{ url: string; drop(): Promise<void> }> {
+  const schema = `t_${Bun.randomUUIDv7().replaceAll("-", "")}`;
+  const admin = await connect(base);
+  await admin.sql.unsafe(`CREATE SCHEMA ${schema}`);
+  const u = new URL(base);
+  u.searchParams.set("options", `-c search_path=${schema}`);
+  return { url: u.toString(), drop: async () => { await admin.sql.unsafe(`DROP SCHEMA ${schema} CASCADE`); await admin.close(); } };
+}