Feature: Rechnungs-System (invoices) — Backend komplett

- DB-Migration: invoices + invoice_items Tabellen inkl. Indizes
- routes/invoices.py: vollständiger Admin-Router (prefix /api/admin/invoices)
  - CRUD: Liste, Detail, Erstellen, Senden, Bezahlen, Stornieren
  - PDF-Generierung via fpdf2 mit §14-UStG-Pflichtangaben (Kleinunternehmer-Hinweis)
  - Cashflow-Übersicht und Quartalsbericht
  - PDF-Download-Endpunkt
  - Speicherung in /scaninput + optionaler Paperless-Upload
- mailer.py: send_email() + Backends um optionale PDF-Anhänge erweitert
  (Brevo: base64, SMTP: MIMEApplication)
- main.py: invoices_router registriert
- docker-compose.yml: /volume1/scaninput:/scaninput Volume hinzugefügt

backend/mailer.py

@@ -6,11 +6,13 @@ Unterstützt zwei Backends (wird automatisch gewählt):
 """
 
 import os
+import base64
 import smtplib
 import asyncio
 import logging
 from email.mime.text import MIMEText
 from email.mime.multipart import MIMEMultipart
+from email.mime.application import MIMEApplication
 
 import httpx
 
@@ -33,9 +35,7 @@ APP_URL   = os.getenv("APP_URL", "https://banyaro.app")
 # ------------------------------------------------------------------
 # Brevo REST-API
 # ------------------------------------------------------------------
-async def _send_brevo(to: str, subject: str, html: str, plain: str):
-    # Absender-Name und -Adresse aus SMTP_FROM parsen
-    # Format: "Ban Yaro <noreply@banyaro.app>" oder "noreply@banyaro.app"
+async def _send_brevo(to: str, subject: str, html: str, plain: str, attachments: list | None = None):
     from_raw = SMTP_FROM
     if "<" in from_raw:
         from_name  = from_raw[:from_raw.index("<")].strip()
@@ -52,6 +52,14 @@ async def _send_brevo(to: str, subject: str, html: str, plain: str):
         "textContent":  plain,
         "headers":      {"X-Mailin-Track-Click": "0", "X-Mailin-Track-Opens": "0"},
     }
+    if attachments:
+        payload["attachment"] = [
+            {
+                "name":    a["filename"],
+                "content": base64.b64encode(a["content"]).decode("ascii"),
+            }
+            for a in attachments
+        ]
     async with httpx.AsyncClient(timeout=15) as client:
         resp = await client.post(
             BREVO_API_URL,
@@ -64,13 +72,25 @@ async def _send_brevo(to: str, subject: str, html: str, plain: str):
 # ------------------------------------------------------------------
 # SMTP Fallback
 # ------------------------------------------------------------------
-def _send_smtp_sync(to: str, subject: str, html: str, plain: str):
-    msg = MIMEMultipart("alternative")
+def _send_smtp_sync(to: str, subject: str, html: str, plain: str, attachments: list | None = None):
+    if attachments:
+        msg = MIMEMultipart("mixed")
+        alt = MIMEMultipart("alternative")
+        alt.attach(MIMEText(plain, "plain", "utf-8"))
+        alt.attach(MIMEText(html, "html", "utf-8"))
+        msg.attach(alt)
+        for a in attachments:
+            part = MIMEApplication(a["content"], Name=a["filename"])
+            part["Content-Disposition"] = f'attachment; filename="{a["filename"]}"'
+            msg.attach(part)
+    else:
+        msg = MIMEMultipart("alternative")
+        msg.attach(MIMEText(plain, "plain", "utf-8"))
+        msg.attach(MIMEText(html,  "html",  "utf-8"))
+
     msg["Subject"] = subject
     msg["From"]    = SMTP_FROM
     msg["To"]      = to
-    msg.attach(MIMEText(plain, "plain", "utf-8"))
-    msg.attach(MIMEText(html,  "html",  "utf-8"))
 
     with smtplib.SMTP(SMTP_HOST, SMTP_PORT, timeout=10) as s:
         s.ehlo()
@@ -83,10 +103,10 @@ def _send_smtp_sync(to: str, subject: str, html: str, plain: str):
 # ------------------------------------------------------------------
 # Öffentliche Funktion
 # ------------------------------------------------------------------
-async def send_email(to: str, subject: str, html: str, plain: str = ""):
+async def send_email(to: str, subject: str, html: str, plain: str = "", attachments: list | None = None):
     if BREVO_API_KEY:
         try:
-            await _send_brevo(to, subject, html, plain)
+            await _send_brevo(to, subject, html, plain, attachments)
             logger.info(f"Mail via Brevo gesendet: «{subject}» → {to}")
             return
         except Exception as e:
@@ -96,7 +116,9 @@ async def send_email(to: str, subject: str, html: str, plain: str = ""):
     if SMTP_HOST:
         loop = asyncio.get_event_loop()
         try:
-            await loop.run_in_executor(None, _send_smtp_sync, to, subject, html, plain)
+            await loop.run_in_executor(
+                None, _send_smtp_sync, to, subject, html, plain, attachments
+            )
             logger.info(f"Mail via SMTP gesendet: «{subject}» → {to}")
             return
         except Exception as e: