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>

web/settings.py

@@ -55,14 +55,6 @@ RETENTION_RUN_INTERVAL_SECONDS: int = int(getenv("RETENTION_RUN_INTERVAL_SECONDS
 LOGIN_RATE_LIMIT: int = int(getenv("LOGIN_RATE_LIMIT", "5"))
 LOGIN_RATE_WINDOW_SECONDS: int = int(getenv("LOGIN_RATE_WINDOW_SECONDS", "900"))
 
-# --- Email (system-wide SMTP for notifications) -------------------------------
-SMTP_HOST: str = getenv("SMTP_HOST", "")
-SMTP_PORT: int = int(getenv("SMTP_PORT", "587"))
-SMTP_USERNAME: str = getenv("SMTP_USERNAME", "")
-SMTP_PASSWORD: str = getenv("SMTP_PASSWORD", "")
-SMTP_FROM: str = getenv("SMTP_FROM", "wohnungsdidi@localhost")
-SMTP_STARTTLS: bool = getenv("SMTP_STARTTLS", "true").lower() in ("true", "1", "yes", "on")
-
 # --- App URL (used to build links in notifications) ---------------------------
 PUBLIC_URL: str = getenv("PUBLIC_URL", "https://flat.lab.moritz.run")