Style outgoing emails instead of plain text (#68)

Every email went through one shared plain-text-only path. Redesigned
send_email/send_test_email around structured paragraphs + an optional
CTA button instead of one pre-formatted string, and render both a
proper styled HTML card (table-based, inline styles -- email clients
strip <style> blocks and don't support CSS variables) and a clean
plain-text fallback from the same input, sent as multipart/alternative.

The HTML is themed per recipient: an email to an existing user renders
in their own selected theme (dark/light/midnight/sunset, or their saved
custom palette), resolved server-side from User.theme/
active_custom_theme_id. Site invites have no account yet to read a
theme from, so they use the default DarkSingularity palette. All five
existing email triggers (site invite, room-added, password reset,
#66's DM notification, admin test email) updated to the new call shape.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/services/site_invite_service.py

@@ -40,13 +40,19 @@ async def create_site_invite(
     await db.refresh(invite)
 
     signup_link = f"{base_url.rstrip('/')}/signup?token={raw_token}"
+    # No theme_user -- the invitee doesn't have an account yet, so there's
+    # no theme of theirs to use (#68). Default palette, same as any
+    # logged-out page.
     await send_email(
         db,
         email,
         "You're invited to join DS Chat",
-        f"You've been invited to join DS Chat by {actor.username}.\n\n"
-        f"Set up your account here:\n{signup_link}\n\n"
-        f"This link expires in 7 days.",
+        [
+            f"You've been invited to join DS Chat by {actor.username}.",
+            "This link expires in 7 days.",
+        ],
+        cta_label="Set up your account",
+        cta_url=signup_link,
     )
     return invite