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/routes/admin.py

@@ -136,7 +136,7 @@ def tab_logs_export(request: Request):
             e["ip"],
         ])
     body = buf.getvalue().encode("utf-8")
-    filename = "wohnungsdidi-protokoll"
+    filename = "lazyflat-protokoll"
     if q.get("from"): filename += f"-{q['from']}"
     if q.get("to"):   filename += f"-bis-{q['to']}"
     filename += ".csv"