feat(api): add RSS/Atom feeds for public mesh data

Four feeds (public messages, node adverts deduped per node, new nodes,
per-community-channel messages), each in RSS 2.0 and Atom, built with
stdlib ElementTree. Feeds are hard-pinned to the logged-out view
(anonymous visibility, spam always filtered, identity headers ignored)
and served at clean public web URLs (/feeds/*) via a shared proxy helper
that forwards If-None-Match + X-Forwarded-* and gates on maintenance
mode. Responses are Redis-cached (ETag/304, public max-age) and
invalidated with messages/adverts/channels mutations. Item/self links
always target the public web endpoint; WEB_PUBLIC_URL pins the canonical
origin. Includes a FEATURE_FEEDS kill switch, SPA autodiscovery <link>
tags, and env/docs updates (v0.21.0 upgrading notes).

docker-compose.yml

@@ -337,6 +337,15 @@ services:
       - REDIS_KEY_PREFIX=${REDIS_KEY_PREFIX:-hub}
       - REDIS_CACHE_TTL=${REDIS_CACHE_TTL:-30}
       - REDIS_CACHE_TTL_DASHBOARD=${REDIS_CACHE_TTL_DASHBOARD:-3600}
+      - REDIS_CACHE_TTL_FEEDS=${REDIS_CACHE_TTL_FEEDS:-300}
+      # RSS/Atom feeds kill switch (bridged from FEATURE_FEEDS so the API, the
+      # web alias, and autodiscovery links track one operator-facing var).
+      - FEEDS_ENABLED=${FEATURE_FEEDS:-true}
+      # Feed titles need the network display name in the API tier too.
+      - NETWORK_NAME=${NETWORK_NAME:-MeshCore Network}
+      # Canonical public site URL for absolute feed links (empty = derive
+      # from the forwarded request host via the web proxy).
+      - WEB_PUBLIC_URL=${WEB_PUBLIC_URL:-}
       - API_CACHE_CONTROL_ENABLED=${API_CACHE_CONTROL_ENABLED:-true}
       # Spam detection: hide-filter switch + threshold (bridged from
       # FEATURE_SPAM_DETECTION so it tracks the collector and web toggle). On by
@@ -440,6 +449,9 @@ services:
       - FEATURE_SPAM_DETECTION=${FEATURE_SPAM_DETECTION:-true}
       # Routes page (route health monitoring) is on by default.
       - FEATURE_ROUTES=${FEATURE_ROUTES:-true}
+      # RSS/Atom feeds (/feeds/* + autodiscovery links); mirrors the API's
+      # FEEDS_ENABLED switch via Compose.
+      - FEATURE_FEEDS=${FEATURE_FEEDS:-true}
     command: ["web"]
     healthcheck:
       test: