feat: ICS-Kalender-Feed für Fristen (Issue #21)

Neuer Endpoint GET /api/fristen/kalender.ics:
- Alle offenen + überfälligen Fristen als ICS-Feed
- Ganztägige Events mit VALUE=DATE
- Überfällige Fristen mit ⚠️-Prefix und Tage-Angabe
- 7-Tage-Vorab-Alarm (VALARM TRIGGER:-P7D)
- Stabile UIDs (frist-{id}@antraege.toppyr.de)
- RFC 5545 konform: CRLF, Line Folding, Escaping
- Cache-Header: public, max-age=3600
- VTIMEZONE Europe/Berlin inkludiert
- Kein Auth erforderlich (öffentlicher Feed)
- Keine externen Dependencies

Closes #21

backend/src/tracker/api/routes/fristen.py

@@ -1,10 +1,11 @@
 from __future__ import annotations
 """API routes for Fristen-Tracking (Issue #17)."""
 
-from datetime import date, datetime
+from datetime import date, datetime, timedelta
 from typing import Optional
 
 from fastapi import APIRouter, Depends, HTTPException, Query
+from fastapi.responses import Response
 from pydantic import BaseModel
 
 from tracker.db.session import get_connection
@@ -171,6 +172,182 @@ def delete_frist(frist_id: int, conn=Depends(_db)):
     return {"ok": True, "deleted": frist_id}
 
 
+@router.get("/kalender.ics")
+def fristen_kalender(conn=Depends(_db)):
+    """ICS-Kalender-Feed für offene und überfällige Fristen."""
+    today = date.today()
+
+    rows = conn.execute(
+        """SELECT f.id, f.typ, f.datum, f.beschreibung, f.status,
+                  k.status AS kette_status,
+                  v_ursprung.aktenzeichen AS kette_aktenzeichen,
+                  v_ursprung.betreff AS kette_betreff
+           FROM fristen f
+           LEFT JOIN ketten k ON f.kette_id = k.id
+           LEFT JOIN vorlagen v_ursprung ON k.ursprung_id = v_ursprung.id
+           WHERE f.status IN ('offen', 'überfällig')
+              OR (f.status = 'offen' AND f.datum < date('now'))
+           ORDER BY f.datum ASC""",
+    ).fetchall()
+
+    events = []
+    for r in rows:
+        row = dict(r)
+        frist_datum = date.fromisoformat(row["datum"]) if row["datum"] else None
+        if not frist_datum:
+            continue
+
+        frist_id = row["id"]
+        typ = row["typ"] or "Frist"
+        beschreibung = row["beschreibung"] or ""
+        aktenzeichen = row["kette_aktenzeichen"] or "–"
+        kette_betreff = row["kette_betreff"] or ""
+        status = row["status"]
+
+        # Detect overdue
+        is_overdue = status == "überfällig" or (status == "offen" and frist_datum < today)
+        if is_overdue:
+            status = "überfällig"
+
+        # Build SUMMARY
+        summary_prefix = "\u26a0\ufe0f " if is_overdue else ""
+        summary = f"{summary_prefix}{typ}: {beschreibung} ({aktenzeichen})"
+
+        # Build DESCRIPTION
+        desc_parts = [beschreibung]
+        if is_overdue:
+            days_overdue = (today - frist_datum).days
+            desc_parts.append(f"\u00dcBERF\u00c4LLIG seit {days_overdue} Tagen")
+        desc_parts.append(f"Kette: {kette_betreff}")
+        desc_parts.append(f"Status: {status}")
+        desc_parts.append("")
+        desc_parts.append("https://antraege.toppyr.de/explorer")
+        description = "\\n".join(desc_parts)
+
+        # CATEGORIES + STATUS mapping
+        if is_overdue:
+            categories = "überfällig"
+            ics_status = "CONFIRMED"
+        else:
+            categories = "offen"
+            ics_status = "TENTATIVE"
+
+        dtstart = frist_datum.strftime("%Y%m%d")
+        dtend = (frist_datum + timedelta(days=1)).strftime("%Y%m%d")
+        dtstamp = datetime.utcnow().strftime("%Y%m%dT%H%M%SZ")
+
+        event_lines = [
+            "BEGIN:VEVENT",
+            f"UID:frist-{frist_id}@antraege.toppyr.de",
+            f"DTSTAMP:{dtstamp}",
+            f"DTSTART;VALUE=DATE:{dtstart}",
+            f"DTEND;VALUE=DATE:{dtend}",
+            f"SUMMARY:{_ics_escape(summary)}",
+            f"DESCRIPTION:{_ics_escape(description)}",
+            "URL:https://antraege.toppyr.de/explorer",
+            f"CATEGORIES:{_ics_escape(categories)}",
+            f"STATUS:{ics_status}",
+            "BEGIN:VALARM",
+            "TRIGGER:-P7D",
+            "ACTION:DISPLAY",
+            f"DESCRIPTION:Frist in 7 Tagen: {_ics_escape(beschreibung)}",
+            "END:VALARM",
+            "END:VEVENT",
+        ]
+        events.append("\r\n".join(event_lines))
+
+    cal_lines = [
+        "BEGIN:VCALENDAR",
+        "VERSION:2.0",
+        "PRODID:-//Antragstracker Hagen//Fristen//DE",
+        "CALSCALE:GREGORIAN",
+        "METHOD:PUBLISH",
+        "X-WR-CALNAME:Antragstracker Fristen",
+        "X-WR-TIMEZONE:Europe/Berlin",
+        "BEGIN:VTIMEZONE",
+        "TZID:Europe/Berlin",
+        "BEGIN:DAYLIGHT",
+        "TZNAME:CEST",
+        "TZOFFSETFROM:+0100",
+        "TZOFFSETTO:+0200",
+        "DTSTART:19700329T020000",
+        "RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=-1SU",
+        "END:DAYLIGHT",
+        "BEGIN:STANDARD",
+        "TZNAME:CET",
+        "TZOFFSETFROM:+0200",
+        "TZOFFSETTO:+0100",
+        "DTSTART:19701025T030000",
+        "RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU",
+        "END:STANDARD",
+        "END:VTIMEZONE",
+    ]
+
+    for event in events:
+        cal_lines.append(event)
+
+    cal_lines.append("END:VCALENDAR")
+
+    ics_body = "\r\n".join(cal_lines) + "\r\n"
+
+    # Apply line folding (max 75 octets per line)
+    ics_body = _ics_fold(ics_body)
+
+    return Response(
+        content=ics_body,
+        media_type="text/calendar; charset=utf-8",
+        headers={"Cache-Control": "public, max-age=3600"},
+    )
+
+
+def _ics_escape(text: str) -> str:
+    """Escape text for ICS content lines (RFC 5545 §3.3.11)."""
+    if not text:
+        return ""
+    text = text.replace("\\", "\\\\")
+    text = text.replace(";", "\\;")
+    text = text.replace(",", "\\,")
+    # Preserve intentional \\n sequences (literal newlines in ICS)
+    # but escape any actual newlines
+    text = text.replace("\n", "\\n")
+    text = text.replace("\r", "")
+    return text
+
+
+def _ics_fold(text: str) -> str:
+    """Fold lines longer than 75 octets per RFC 5545 §3.1."""
+    result = []
+    for line in text.split("\r\n"):
+        encoded = line.encode("utf-8")
+        if len(encoded) <= 75:
+            result.append(line)
+            continue
+        # Fold: first line 75 octets, continuations 74 (space prefix)
+        chunks = []
+        while len(encoded) > 75:
+            # Cut at 75 octets for first chunk, 74 for subsequent
+            cut = 75 if not chunks else 74
+            # Don't break in middle of multi-byte UTF-8
+            split_at = cut
+            while split_at > 0 and (encoded[split_at] & 0xC0) == 0x80:
+                split_at -= 1
+            chunk = encoded[:split_at].decode("utf-8")
+            encoded = encoded[split_at:]
+            if chunks:
+                chunks.append(" " + chunk)
+            else:
+                chunks.append(chunk)
+        # Remaining
+        remaining = encoded.decode("utf-8")
+        if remaining:
+            if chunks:
+                chunks.append(" " + remaining)
+            else:
+                chunks.append(remaining)
+        result.append("\r\n".join(chunks))
+    return "\r\n".join(result)
+
+
 @router.get("/ueberfaellig")
 def get_ueberfaellige(conn=Depends(_db)):
     """Überfällige Fristen abrufen und automatisch Status setzen."""