fix(journal): fail loud in production when secrets are unset

Adds `requireSecret(name, devFallback)` in lib/config.server.ts and
`getDatabaseUrl()` in @trails-cool/db. Both:
- return the env var when set,
- fall back to the dev default in non-production,
- throw at boot in production if the env var is missing OR matches the
  known dev fallback (which would otherwise silently ship a public
  secret / point at localhost).

Applied to:
- JWT_SECRET (lib/jwt.server.ts) — was `?? "dev-jwt-secret-change-in-production"`
- SESSION_SECRET (lib/auth/session.server.ts) — was `?? "dev-secret-change-in-production"`
- DATABASE_URL (server.ts health + boss; packages/db migrate-data) — was
  `?? "postgres://trails:trails@localhost:5432/trails"`

Why: these strings are in the repo and known to attackers. A
misconfigured prod deploy that forgot to set them would either run with
guessable signing keys (full session/JWT forgery) or connect to a
non-existent localhost DB. Better to refuse to start than to silently
operate insecurely.

Tests:
- packages/db/src/get-database-url.test.ts (5 cases)
- lib/config.server.test.ts gains `requireSecret` cases (4 new)

Full repo: pnpm typecheck, pnpm lint, pnpm test all green.

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

packages/db/src/migrate-data.ts

@@ -10,13 +10,13 @@ import { readdirSync, readFileSync } from "node:fs";
 import path from "node:path";
 import { fileURLToPath } from "node:url";
 import postgres from "postgres";
+import { getDatabaseUrl } from "./index.ts";
 
 const __dirname = path.dirname(fileURLToPath(import.meta.url));
 const migrationsDir = path.resolve(__dirname, "..", "migrations");
 
 async function main() {
-  const url = process.env.DATABASE_URL ?? "postgres://trails:trails@localhost:5432/trails";
-  const sql = postgres(url);
+  const sql = postgres(getDatabaseUrl());
   try {
     const files = readdirSync(migrationsDir)
       .filter((f) => f.endsWith(".sql"))