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>

web/templates/_layout.html

@@ -8,7 +8,7 @@
     <div class="max-w-6xl mx-auto px-6 py-3 flex items-center justify-between">
         <div class="flex items-center gap-3">
             <span class="brand-dot"><img src="/static/didi.webp" alt=""></span>
-            <h1 class="text-xl font-semibold">wohnungsdidi</h1>
+            <h1 class="text-xl font-semibold">lazyflat</h1>
         </div>
         <div class="flex items-center gap-4 text-sm">
             <span class="text-slate-500">{{ user.username }}{% if is_admin %} · <span class="chip chip-info">Administrator</span>{% endif %}</span>