feat: Add OIDC authentication and SMTP email notifications

[KeplerAeroIT]

Summary

• OIDC / SSO authentication — adds a "Login with SSO" button to the login page, a /auth/callback route, and full OIDC session handling. Tested with Authentik; should work with any compliant OIDC provider. Supports optional auto-provisioning of new accounts on first SSO login.
• SMTP email notifications — adds a "Send via Email" button on invoice detail pages. Configurable with any SMTP server (Gmail, Outlook, self-hosted, etc.). The invoice detail page shows the email delivery status after sending.
• Templates — added two more templates for invoices

Changes

Area	Files	Description
OIDC	backend/src/utils/oidc.ts, backend/src/routes/auth.ts	OIDC provider integration and callback handling
OIDC	frontend/src/routes/auth/callback/+server.ts, login/+page.svelte	Frontend callback route and SSO login button
Email	backend/src/utils/email.ts, backend/src/routes/admin.ts	SMTP client and send-invoice-by-email endpoint
Email	frontend/src/routes/invoices/[id]/+page.svelte, +page.server.ts	Send email action and delivery status indicator
DB	backend/src/database/init.ts, migrations.sql	Schema additions for OIDC and email tracking
Config	.env.example	Documented all new SMTP and OIDC environment variables

Test plan

• Configure OIDC credentials in .env, verify SSO login redirects and callback creates a valid session
• Verify standard password login still works when OIDC is not configured (OIDC_ENABLED unset or false)
• Configure SMTP credentials, open an invoice, send via email, verify delivery and status indicator update
• Run with no SMTP config and confirm the send button is absent / gracefully disabled

[kittendevv]

Hi in your PR you list you have added two new invoice templates but when looking through your commits I don't see them anywhere.

You also provided a test checklist, have you tested the things you list there.

Anyway thank you for creating a PR! Would be happy to merge this.

[KeplerAeroIT]

Hey, sorry. I forgot the push the Templates update. I've pushed it now.

I have independently verified and tested everything.

[kittendevv]

Ah, no worries! Everything looks good so I am gonna go ahead and merge this.

Thank you for your contribution!