Add archive scraping, webcal feed, and scrape schedule reporting

- scraper.py: --archive flag to scrape https://fundbureau.de/archiv.html
  (past events), fixing a container-specific wait selector and a crash
  on events with no door time that this surfaced.
- api: EventStore now merges an optional archive JSON file into the
  main event list (deduped by date+name); adds GET /events/archive.
- api: GET /calendar.ics serves an RFC 5545 feed of all events for
  webcal subscriptions, with an all-day fallback when no door time is
  parseable.
- api: GET /health reports last-scrape time (from file mtime) and, via
  new SCRAPE_CRON/ARCHIVE_SCRAPE_CRON env vars, the next scheduled run
  and seconds until it, for both the regular and archive scrape.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

api/scrape_schedule.py

@@ -0,0 +1,58 @@
+"""Reports when a scrape last ran and when cron will next run it.
+
+"Last ran" is read from the mtime of the scraper's output file - true
+regardless of how/when the scrape was actually invoked. "Next run" requires
+telling this module the cron schedule via an env var (there's no reliable,
+portable way to introspect another process's crontab from here), given as
+a standard 5-field cron expression, e.g. "0 * * * *".
+"""
+
+from datetime import datetime
+from pathlib import Path
+from typing import Optional
+
+from croniter import croniter
+
+
+def _now_local() -> datetime:
+    return datetime.now().astimezone()
+
+
+def _file_mtime_local(path: Path) -> Optional[datetime]:
+    try:
+        return datetime.fromtimestamp(path.stat().st_mtime).astimezone()
+    except FileNotFoundError:
+        return None
+
+
+def _next_cron_run(cron_expr: str, after: datetime) -> Optional[datetime]:
+    try:
+        naive_after = after.replace(tzinfo=None)
+        return croniter(cron_expr, naive_after).get_next(datetime).astimezone()
+    except (ValueError, KeyError):
+        return None
+
+
+def get_schedule_info(path: Path, cron_expr: Optional[str]) -> dict:
+    """Build the last/next-scrape block for one scraper output file.
+
+    ``cron_expr`` is that scrape's cron expression (e.g. "0 * * * *");
+    None/invalid -> next-run fields are null.
+    """
+    now = _now_local()
+    cron_expr = (cron_expr or "").strip() or None
+
+    last_scrape_at = _file_mtime_local(path)
+    next_scrape_at = _next_cron_run(cron_expr, now) if cron_expr else None
+
+    return {
+        "cron_schedule": cron_expr,
+        "last_scrape_at": last_scrape_at.isoformat() if last_scrape_at else None,
+        "seconds_since_last_scrape": (
+            round((now - last_scrape_at).total_seconds()) if last_scrape_at else None
+        ),
+        "next_scrape_at": next_scrape_at.isoformat() if next_scrape_at else None,
+        "seconds_until_next_scrape": (
+            round((next_scrape_at - now).total_seconds()) if next_scrape_at else None
+        ),
+    }