refactor: rename wohnungsdidi → lazyflat

Container names, FastAPI titles, email subjects, filenames, brand text,
session cookie, User-Agent, docstrings, README. Volume lazyflat_data and
/data/lazyflat.sqlite already used the new name, so on-disk data is
preserved; dropped the now-obsolete legacy-rename comments.

Side effect: SESSION_COOKIE_NAME change logs everyone out on deploy.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -1,7 +1,7 @@
 services:
   web:
     build: ./web
-    container_name: wohnungsdidi-web
+    container_name: lazyflat-web
     restart: unless-stopped
     depends_on:
       apply:
@@ -25,15 +25,13 @@ services:
       - ANTHROPIC_API_KEY=${ANTHROPIC_API_KEY:-}
       - ANTHROPIC_MODEL=${ANTHROPIC_MODEL:-claude-haiku-4-5-20251001}
     volumes:
-      # Legacy volume name — kept so Coolify reuses the existing data volume
-      # after the rename from lazyflat → wohnungsdidi.
       - lazyflat_data:/data
     expose:
       - "8000"
 
   apply:
     build: ./apply
-    container_name: wohnungsdidi-apply
+    container_name: lazyflat-apply
     restart: unless-stopped
     expose:
       - "8000"
@@ -47,7 +45,7 @@ services:
 
   alert:
     build: ./alert
-    container_name: wohnungsdidi-alert
+    container_name: lazyflat-alert
     restart: unless-stopped
     depends_on:
       web: