fix(email): give every mail an HTML alternative so its link is clickable

The bodies are text/plain only, so whether the URL in a verification or
password-reset mail is a link at all was the client's decision. Gmail, Outlook and
Apple Mail linkify plain text; lighter web clients render the body verbatim and leave
the link as dead text — in a message whose entire purpose is that one link.

_send now attaches an HTML part generated from the SAME plain body, rather than
per-template HTML that would have to be kept in sync with it: one converter, all
eleven templates, and text/plain stays the primary part and the source of truth.
Escaping runs before linkification — bodies interpolate untrusted values (a
requester's address, the operator contact) — so no caller can inject markup, and a
query string's `&` is already `&` when it reaches the href, which is what an HTML
attribute wants. Sentence-final punctuation is trimmed out of the href while
url-safe token characters (`-`, `_`) are left alone. Inline styles only, no images,
no external CSS, no tracking.

backend/app/email.py

@@ -10,7 +10,9 @@ verification links, password resets and access-request notices, and the operator
 to be a person telling them nothing arrived.
 """
 
+import html
 import logging
+import re
 import smtplib
 import ssl
 from email.message import EmailMessage
@@ -105,6 +107,48 @@ def _admin_contact() -> str | None:
     return next(iter(sorted(settings.admin_email_set)), None)
 
 
+# A bare URL in a text/plain mail is only clickable if the CLIENT decides to linkify it. Gmail,
+# Outlook and Apple Mail do; plenty of lighter web clients render the body verbatim and leave the
+# link as dead text — which, for a verification or password-reset mail, is the whole message. So
+# every mail also carries an HTML alternative built from the SAME plain body (below), rather than
+# per-template HTML that would have to be kept in sync with it.
+#
+# `[^\s<]` stops at whitespace and at the `<` of any following markup; the trailing-punctuation trim
+# keeps a sentence-final period out of the href. `;` and `:` are deliberately NOT trimmed — escaping
+# turns `&` into `&amp;`, so a query string can legitimately end in a semicolon.
+_URL_RE = re.compile(r"https?://[^\s<]+")
+
+
+def _html_body(text: str) -> str:
+    """Render a plain-text mail body as minimal HTML: escaped, blank-line-separated paragraphs, with
+    real links. Escaping happens FIRST and the linkifier then runs over the escaped text — bodies
+    interpolate untrusted values (a requester's email address, the operator contact), so no caller
+    can inject markup, and `&` inside a URL is already `&amp;` by the time it reaches the href, which
+    is what an HTML attribute wants anyway."""
+
+    def _anchor(m: re.Match[str]) -> str:
+        url = m.group(0)
+        trail = ""
+        while url and url[-1] in ".,!?)":
+            url, trail = url[:-1], url[-1] + trail
+        if not url:
+            return trail
+        return f'<a href="{url}">{url}</a>{trail}'
+
+    paragraphs = [p for p in html.escape(text).split("\n\n") if p.strip()]
+    rendered = "".join(
+        f"<p>{_URL_RE.sub(_anchor, p).replace(chr(10), '<br>')}</p>" for p in paragraphs
+    )
+    # Inline styles only — many clients drop <style> blocks. No images, no external CSS, no tracking:
+    # the HTML part exists to make the link clickable, not to become a template to maintain.
+    return (
+        '<html><body style="margin:0;padding:16px;font-family:system-ui,-apple-system,'
+        "'Segoe UI',sans-serif;font-size:15px;line-height:1.6\">"
+        f'<div style="max-width:560px">{rendered}</div>'
+        "</body></html>"
+    )
+
+
 def _send(to: list[str], subject: str, body: str, reply_to: str | None = None) -> bool:
     recipients = [e for e in to if e]
     if not recipients:
@@ -123,7 +167,10 @@ def _send(to: list[str], subject: str, body: str, reply_to: str | None = None) -
     msg["Date"] = formatdate(localtime=True)
     if reply_to:
         msg["Reply-To"] = reply_to
+    # text/plain stays the primary part (it is the source of truth, and a text-only client still
+    # gets the full message); the HTML alternative below is what makes the link clickable everywhere.
     msg.set_content(body)
+    msg.add_alternative(_html_body(body), subtype="html")
     try:
         with smtplib.SMTP(c["host"], c["port"], timeout=20) as s:
             s.starttls(context=ssl.create_default_context())