Add SMTP send_mail SQL function with configuration and lettre integration

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## unreleased
 
+- **SQLPage can now send plain-text email through an SMTP relay.** Configure the relay and optional authentication with `smtp_host`, `smtp_port`, `smtp_username`, `smtp_password`, `smtp_from`, and `smtp_tls_mode`, then call `sqlpage.send_mail` with a JSON message. It returns `{"status":"accepted"}` on SMTP acceptance or `{"status":"error","error_code":"...","error":"..."}` without stopping the request; SQL `NULL` is passed through without sending. Messages support multiple `to` and `cc` recipients, reply-to addresses, and data-URL attachments with a configurable combined decoded-size limit. SMTP passwords are redacted from startup debug logs.
 - **SQLPage functions can now be composed with database results.** Direct calls such as `SELECT sqlpage.url_encode(url) FROM links` already ran once per row. Per-row evaluation now also works through parentheses, concatenation, `COALESCE`, JSON constructors, and nested SQLPage functions. The database first decides which rows exist, then SQLPage evaluates the selected expression for each row. This enables patterns that were not previously possible, such as fetching only missing cached values or rendering a reusable SQL file with parameters from each row:
 
   ```sql

Cargo.toml

@@ -79,6 +79,14 @@ openidconnect = { version = "4.0.0", default-features = false, features = ["acce
 encoding_rs = "0.8.35"
 odbc-sys = { version = "0", optional = true }
 regex = "1"
+lettre = { version = "0.11", default-features = false, features = [
+    "aws-lc-rs",
+    "builder",
+    "rustls-native-certs",
+    "smtp-transport",
+    "tokio1-rustls",
+    "webpki-roots",
+] }
 
 # OpenTelemetry / tracing
 tracing = "0.1"

configuration.md

@@ -41,6 +41,13 @@ Here are the available configuration options and their default values:
 | `environment`                                 | development                                                 | The environment in which SQLPage is running. Can be either `development` or `production`. In `production` mode, SQLPage will hide error messages and stack traces from the user, and will cache sql files in memory to avoid reloading them from disk. |
 | `cache_stale_duration_ms`                     | 1000 (prod), 0 (dev)                                        | The duration in milliseconds that a file can be cached before its freshness is checked against the filesystem. Defaults to 1000ms (1 second) in production and 0ms in development. |
 | `content_security_policy`                     | `script-src 'self' 'nonce-{NONCE}'`                          | The [Content Security Policy](https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP) to set in the HTTP headers. If you get CSP errors in the browser console, you can set this to the empty string to disable CSP. If you want a custom CSP that contains a nonce, include the `'nonce-{NONCE}'` directive in your configuration string and it will be populated with a random value per request.                                                                                                           |
+| `smtp_host`                                  |                                                              | SMTP server host used by the `sqlpage.send_mail` function. Set with `SMTP_HOST` in the environment. |
+| `smtp_port`                                  | 25 (`none`), 465 (`tls`), or 587 (`starttls`)                | SMTP server port. The default depends on `smtp_tls_mode`. Set this explicitly for relays using a nonstandard port. |
+| `smtp_username`                              |                                                              | Optional SMTP user name for `sqlpage.send_mail`. When set, SQLPage authenticates to `SMTP_HOST` using this user name and `smtp_password`. Credentials require `smtp_tls_mode` to be `starttls` or `tls`. |
+| `smtp_password`                              |                                                              | Optional SMTP password for `sqlpage.send_mail`. `smtp_username` and `smtp_password` must be configured together. |
+| `smtp_from`                                  |                                                              | Default sender address for `sqlpage.send_mail`, optionally including a display name. Individual messages can override it with their `from` property. |
+| `smtp_tls_mode`                              | `starttls`                                                   | Encryption mode for `sqlpage.send_mail`: `starttls` requires a STARTTLS upgrade, `tls` uses TLS from connection start, and `none` permits plaintext only without credentials for trusted local SMTP servers. |
+| `max_email_attachment_size`                  | 10485760                                                     | Maximum combined decoded size, in bytes, of all attachments in one email. Defaults to 10 MiB. This is independent of `max_uploaded_file_size` because attachments may come from sources other than form uploads. |
 | `system_root_ca_certificates`                 | false                                                      | Whether to use the system root CA certificates to validate SSL certificates when making http requests with `sqlpage.fetch`. If set to false, SQLPage will use its own set of root CA certificates. If the `SSL_CERT_FILE` or `SSL_CERT_DIR` environment variables are set, they will be used instead of the system root CA certificates. |
 | `max_recursion_depth`                         | 10                                                           | Maximum depth of recursion allowed in the `run_sql` function. Maximum value is 255. |
 | `markdown_allow_dangerous_html`               | false                                                        | Whether to allow raw HTML in markdown content. Only enable this if the markdown content is fully trusted (not user generated). |