auto-assign filter priority (max+10) on create

New filter rules no longer default to priority 100. Server picks
max(priority)+10 per account when client sends priority=None (or 10
if the account has no rules yet), and the UI pre-fills the input
with the next value instead of a fixed 100. Avoids ambiguous ordering
when multiple rules end up with the same priority.

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

app/schemas/schemas.py

@@ -41,7 +41,7 @@ class FilterActionResponse(FilterActionCreate):
 class FilterRuleCreate(BaseModel):
     name: str
     account_id: int
-    priority: int = 100
+    priority: int | None = None  # None → automatisch ans Ende einsortieren (max+10)
     enabled: bool = True
     stop_processing: bool = False
     source_folder: str = "INBOX"