chore: sweep dead code across all three services

Per review §1 — verified no callers before each deletion:

- _next_scrape_utc (context dict key never read by any template)
- ALERT_SCRAPE_INTERVAL_SECONDS settings import (only _next_scrape_utc read it)
- alert/paths.py (imported by nothing)
- alert/settings.py LANGUAGE (alert doesn't use translations.toml)
- alert/main.py: the vestigial `c = {}` connectivity dict, the comment
  about re-enabling it, and the entire connectivity block in
  _flat_payload — the web-side columns stay NULL on insert now
- alert/maps.py: DESTINATIONS, calculate_score, _get_next_weekday,
  _calculate_transfers (only geocode is used in the scraper)
- alert/flat.py: connectivity + display_address properties,
  _connectivity field, unused datetime import
- apply/utils.py str_to_preview (no callers) — file removed
- web/matching.py: max_morning_commute + commute check
- web/app.py: don't pass connectivity dict into flat_matches_filter,
  don't write email_address through update_notifications
- web/db.py: get_error (no callers); drop kill_switch,
  max_morning_commute, email_address from their allowed-sets so they're
  not writable through update_* anymore
- web/settings.py + docker-compose.yml: SMTP_HOST/PORT/USERNAME/PASSWORD/
  FROM/STARTTLS (notifications.py is telegram-only now)

DB columns themselves (kill_switch, email_address, max_morning_commute,
connectivity_morning_time, connectivity_night_time) stay in the schema
— SQLite can't drop them cheaply and they're harmless.

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

alert/settings.py

@@ -13,7 +13,6 @@ def _required(key: str) -> str:
     return val
 
 
-LANGUAGE: str = getenv("LANGUAGE", "en")
 TIME_INTERVALL: int = int(getenv("SLEEP_INTERVALL", "60"))
 
 # web backend: alert POSTs discovered flats here