fix(journal): centralize session-auth helpers + extract .server.ts siblings

Follow-up to PR #406 — addresses the two items deferred from the audit:

#7 — Centralize auth helpers
- New `requireSessionUser(request)` in lib/auth/session.server.ts that
  returns the user or throws a redirect to /auth/login.
- New `requireSessionUserJson(request)` companion that throws a 401 JSON
  response (for fetcher/JSON endpoints).
- Replace the repeated
    const user = await getSessionUser(request);
    if (!user) return redirect("/auth/login");
  pattern across 18 route loaders/actions. Removes the duplicated guard
  preamble and gives a single chokepoint to evolve later (e.g., for
  terms-version gating).

#8 — Extract heavy loaders into .server.ts siblings
- routes/home.tsx → home.server.ts (DB count query + listActivities +
  listRecentPublicActivities)
- routes/users.$username.tsx → users.$username.server.ts (user lookup +
  follow state + counts + listPublicRoutes/Activities + persona check)
- routes/settings.connections.tsx → settings.connections.server.ts
  (connected_services join + manifest merge)

Each route file shrinks to a thin delegator: `loader` calls
`loadXxx(request)`. The component module no longer transitively pulls
`getDb` and Drizzle schema into its import graph — Vite's tree-shake
already strips server-only code from the client bundle, but the
explicit `.server.ts` suffix makes that contract local and auditable.

Other 17 routes that mix loader/action with components are left as-is
for now: they're each small enough that the split adds churn without
buying much clarity. The pattern is documented by the three examples;
the rest can convert opportunistically when they grow.

Tests:
- lib/auth/session.server.test.ts (4 cases — redirect for missing
  cookie, redirect for ghost userId, success path, JSON 401 variant)

Full repo: pnpm typecheck, pnpm lint, pnpm test all green
(181 passed | 31 integration-gated skipped).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/journal/app/routes/api.sync.komoot.import.ts

@@ -3,17 +3,16 @@
 // Returns { batchId } immediately; poll /api/sync/komoot/import-status for progress.
 
 import { randomUUID } from "node:crypto";
-import { data, redirect } from "react-router";
+import { data } from "react-router";
 import type { Route } from "./+types/api.sync.komoot.import";
-import { getSessionUser } from "~/lib/auth/session.server";
+import { requireSessionUser } from "~/lib/auth/session.server";
 import { getService } from "~/lib/connected-services/manager";
 import { getBoss } from "~/lib/boss.server";
 import { getDb } from "~/lib/db";
 import { importBatches } from "@trails-cool/db/schema/journal";
 
 export async function action({ request }: Route.ActionArgs) {
-  const user = await getSessionUser(request);
-  if (!user) return redirect("/auth/login");
+  const user = await requireSessionUser(request);
 
   const service = await getService(user.id, "komoot");
   if (!service) return data({ error: "not_connected" }, { status: 400 });