feat(profile): profile pages on the account domain, and links verified by rel="me"

A profile now has a page on the web, at the root of the account domain
(https://halkyon.eu/@raisondetredev where the operator set ACCOUNT_DOMAIN,
else on the instance's web server), so the address belongs to the
identity rather than to Learn and stays when other services share it
(ADR 0011). The page shows what the app's profile sheet shows, in English
or German, without JavaScript or cookies, asks search engines not to list
it, marks every link rel="me" so other sites can verify links to it, and
sends a visitor who asked on another host to its own address. Caddy
forwards /@… to the web server, and serves nothing else on the account
domain; compose now passes ACCOUNT_DOMAIN on, which it never did.

A link is verified as Mastodon verifies one: its page links back to the
profile's with rel="me". profile.checkLinks fetches each link's page, at
most ten times an hour, over HTTPS on port 443 and to public addresses
only, connecting to the address it checked and verifying the certificate
for the host, with three redirects, 1 MiB and ten seconds at most, and
keeps what links back on the profile (verifiedLinks, a column of a table
no device syncs). A changed link loses its check, a new handle all of
them. The app shows the page's address to copy, checks a link saved by
itself and on Check links, and marks verified links in the form and in
others' profiles; a check answered after a newer save only adds its
checks.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

deploy/compose.yaml

@@ -36,6 +36,9 @@ services:
       # `learn.halkyon.eu` on its server (automatic Let's Encrypt certificate);
       # `http://localhost` for the local rehearsal (plain HTTP, no certificate).
       DOMAIN: ${DOMAIN:?set DOMAIN in deploy/.env}
+      # The account domain whose profile pages this instance serves, where it isn't
+      # DOMAIN; empty for none.
+      ACCOUNT_SITE: ${ACCOUNT_DOMAIN:-}
     volumes:
       - ./Caddyfile:/etc/caddy/Caddyfile:ro
       - caddy_data:/data # certificates: must survive restarts (rate limits)
@@ -74,6 +77,8 @@ services:
       SERVERPOD_PASSWORD_emailFrom: ${EMAIL_FROM:-}
       # Where the source of what this instance runs is; empty: the project's.
       SOURCE_URL: ${SOURCE_URL:-}
+      # The domain people's handles are on; empty: DOMAIN's host.
+      ACCOUNT_DOMAIN: ${ACCOUNT_DOMAIN:-}
       # The Jellyfin server suggested to people; empty: jellyfin. on the
       # instance's own domain, if one answers there; `none`: none.
       JELLYFIN_URL: ${JELLYFIN_URL:-}