docs: add mail configuration guide

Covers dev (Mailpit), production SMTP, all env vars with defaults,
common provider settings, and how to disable mail entirely.

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

docs/mail.md

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+# Mail configuration
+
+Familienarchiv uses Spring Mail to send password reset emails. The mail sender is **optional** — if no SMTP host is configured, the feature degrades gracefully: a reset token is still created in the database, but no email is sent and a warning is logged.
+
+## How it works in each environment
+
+| Environment | Default behaviour |
+|---|---|
+| `docker-compose up` (dev) | Mailpit catches all emails — nothing leaves your machine |
+| CI | No mail host set — emails are silently skipped, tokens tested via the `/api/auth/reset-token-for-test` endpoint |
+| Production | Real SMTP server configured via environment variables |
+
+---
+
+## Development — Mailpit
+
+[Mailpit](https://github.com/axllent/mailpit) is included in `docker-compose.yml` as a local mail catcher. It accepts SMTP connections from the backend and displays all caught emails in a web inbox. No credentials or external network access required.
+
+**Start the stack as usual:**
+
+```bash
+docker-compose up -d
+```
+
+**Open the inbox:**
+
+```
+http://localhost:8025
+```
+
+All password reset emails appear here. Copy the reset link from the email body and open it in your browser to complete the flow end-to-end locally.
+
+**Ports (configurable in `.env`):**
+
+| Variable | Default | Purpose |
+|---|---|---|
+| `PORT_MAILPIT_UI` | `8025` | Mailpit web inbox |
+| `PORT_MAILPIT_SMTP` | `1025` | SMTP port (used internally by the backend) |
+
+---
+
+## Production — real SMTP
+
+To send real emails, set the following variables in your `.env` file (or as host environment variables). The `MAIL_HOST` variable is the switch — leaving it empty disables outgoing mail entirely.
+
+```dotenv
+# Required
+APP_BASE_URL=https://your-domain.example.com   # Base URL inserted into reset links
+MAIL_HOST=smtp.example.com
+MAIL_PORT=587
+MAIL_USERNAME=your-smtp-user
+MAIL_PASSWORD=your-smtp-password
+
+# Optional — adjust if your provider uses different settings
+MAIL_SMTP_AUTH=true          # default: false (Mailpit needs false)
+MAIL_STARTTLS_ENABLE=true    # default: false (Mailpit needs false)
+APP_MAIL_FROM=noreply@your-domain.example.com
+```
+
+**Common provider settings:**
+
+| Provider | Host | Port | Auth | STARTTLS |
+|---|---|---|---|---|
+| Gmail (App Password) | `smtp.gmail.com` | `587` | `true` | `true` |
+| Mailgun | `smtp.mailgun.org` | `587` | `true` | `true` |
+| Hetzner | `mail.your-server.de` | `587` | `true` | `true` |
+| Self-hosted Postfix | your server IP/hostname | `587` | `true` | `true` |
+
+> **Gmail note:** You must use an [App Password](https://support.google.com/accounts/answer/185833), not your regular account password. 2-Step Verification must be enabled on the account.
+
+---
+
+## Environment variable reference
+
+All variables have safe defaults so the app starts without any mail configuration.
+
+| Variable | Default (docker-compose) | Description |
+|---|---|---|
+| `MAIL_HOST` | `mailpit` | SMTP hostname. Empty string disables mail entirely. |
+| `MAIL_PORT` | `1025` | SMTP port. |
+| `MAIL_USERNAME` | *(empty)* | SMTP username. Leave empty if your server needs no auth. |
+| `MAIL_PASSWORD` | *(empty)* | SMTP password. |
+| `MAIL_SMTP_AUTH` | `false` | Enable SMTP authentication (`true` for real servers). |
+| `MAIL_STARTTLS_ENABLE` | `false` | Enable STARTTLS (`true` for real servers on port 587). |
+| `APP_MAIL_FROM` | `noreply@familienarchiv.local` | The `From:` address on outgoing emails. |
+| `APP_BASE_URL` | `http://localhost:3000` | Base URL prepended to password reset links. |
+
+---
+
+## Disabling mail entirely
+
+Set `MAIL_HOST` to an empty string. Spring Boot will not create a mail sender bean and no emails will be sent. Password reset tokens are still written to the database — useful if you want to test the reset flow via the API directly.
+
+```dotenv
+MAIL_HOST=
+```