chore: rebrand Subfeed -> Siftlode

Rename all user-facing references (UI wordmark Sift+lode, titles, app name,
legal pages, onboarding wizard, emails, README/docs) and infra paths
(/srv/subfeed -> /srv/siftlode, image tag, deploy script, backup filenames).

Internal identifiers kept on purpose: Postgres user/db "subfeed", logger
namespace, localStorage keys, and the subfeed_pgdata volume (renaming would
orphan the migrated production data).

docker-compose.prod.yml

@@ -6,11 +6,11 @@
 # has a hard memory/CPU cap so the stack can't exhaust the small VPS.
 #
 # Secrets live OUTSIDE the repo, in an env file on the host. Deploy with:
-#   docker compose -f docker-compose.prod.yml --env-file /srv/subfeed/.env up -d --build
+#   docker compose -f docker-compose.prod.yml --env-file /srv/siftlode/.env up -d --build
 #
 # Required keys in that env file: POSTGRES_PASSWORD, SECRET_KEY (>=32 chars),
 # TOKEN_ENCRYPTION_KEY (Fernet), GOOGLE_CLIENT_ID, GOOGLE_CLIENT_SECRET,
-# OAUTH_REDIRECT_URL=https://subfeed.b1fr0st.eu/auth/callback, ALLOWED_EMAILS, ADMIN_EMAILS.
+# OAUTH_REDIRECT_URL=https://siftlode.b1fr0st.eu/auth/callback, ALLOWED_EMAILS, ADMIN_EMAILS.
 
 services:
   db:
@@ -37,9 +37,9 @@ services:
     build:
       context: .
       dockerfile: Dockerfile
-    image: subfeed-api:local
+    image: siftlode-api:local
     env_file:
-      - /srv/subfeed/.env
+      - /srv/siftlode/.env
     environment:
       DATABASE_URL: postgresql+psycopg://${POSTGRES_USER:-subfeed}:${POSTGRES_PASSWORD}@db:5432/${POSTGRES_DB:-subfeed}
       # The public instance owns the background scheduler (single writer).