rename to wohnungsdidi + didi logo + footer for all + seconds-only counter

- App is now called "wohnungsdidi" everywhere user-facing (page title,
  nav brand, login header, notification subjects, report filename,
  FastAPI titles, log messages)
- Brand dot replaced with an image of Didi (web/static/didi.webp),
  rendered as a round 2.25rem avatar in _layout + login
- "Programmiert für Annika ♥" footer now shows for every logged-in user,
  not only Annika
- Count-up shows only seconds ("vor 73 s") regardless of age — no
  rollover to minutes/hours
- Data continuity: DB file stays /data/lazyflat.sqlite and the Docker
  volume stays lazyflat_data so the rename doesn't strand existing data
- Session cookie renamed to wohnungsdidi_session (one-time logout on
  rollout)

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

web/enrichment.py

@@ -127,7 +127,7 @@ def _download_images(flat_id: str, urls: list[str], referer: str) -> int:
             r = requests.get(
                 raw_url,
                 headers={"Referer": referer,
-                         "User-Agent": "Mozilla/5.0 (lazyflat enricher)"},
+                         "User-Agent": "Mozilla/5.0 (wohnungsdidi enricher)"},
                 timeout=IMAGE_TIMEOUT,
                 stream=True,
             )