feat: add recurring invoices, public links, time tracker, payments, reminders

- Recurring invoices: schedule-based auto-generation with CRUD UI at /dashboard/invoices/recurring and POST /api/cron/generate-recurring cron endpoint
- Public invoice link: generate/revoke shareable /i/[token] page for unauthenticated clients with PDF download
- Live time tracker: localStorage-persisted timer widget in invoice editor that appends a line item on stop (rounds to nearest 0.25h)
- Partial payment tracking: record payments per invoice, auto-mark paid when fully covered, balance due display
- Send reminder: email reminder via Resend with custom message dialog and last-sent indicator

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

src/env.js

@@ -20,6 +20,7 @@ export const env = createEnv({
       .default("development"),
     DB_DISABLE_SSL: z.coerce.boolean().optional(),
     DISABLE_SIGNUPS: z.coerce.boolean().optional(),
+    CRON_SECRET: z.string().optional(),
     // SSO / Authentik (optional)
     AUTHENTIK_ISSUER: z.string().url().optional(),
     AUTHENTIK_CLIENT_ID: z.string().optional(),
@@ -85,6 +86,7 @@ export const env = createEnv({
     AUTHENTIK_CLIENT_ID: process.env.AUTHENTIK_CLIENT_ID,
     AUTHENTIK_CLIENT_SECRET: process.env.AUTHENTIK_CLIENT_SECRET,
     AUTHENTIK_ORIGIN: process.env.AUTHENTIK_ORIGIN,
+    CRON_SECRET: process.env.CRON_SECRET,
     NEXT_PUBLIC_APP_URL: process.env.NEXT_PUBLIC_APP_URL,
     NEXT_PUBLIC_UMAMI_WEBSITE_ID: process.env.NEXT_PUBLIC_UMAMI_WEBSITE_ID,
     NEXT_PUBLIC_UMAMI_SCRIPT_URL: process.env.NEXT_PUBLIC_UMAMI_SCRIPT_URL,