feat: custom date range filter on calendar page

- Backend: /api/eco/calendar accepts date_from + date_to query params
  when period=custom; get_calendar() uses them directly without override
  Limit raised to 5000 for wide date ranges
- Frontend: "Custom" tab at end of period list; shows two date inputs
  (from/to) with Apply button; displays day count; fetches on Apply click
  (not on every keystroke to avoid hammering the API)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/routers/eco.py

@@ -421,10 +421,12 @@ def ff_scrape_status_ep() -> Dict[str, Any]:
 
 @router.get("/calendar")
 def ff_calendar(
-    period:     str            = Query("recent", description="recent|today|tomorrow|yesterday|this_week|next_week|previous_week|this_month|next_month|previous_month"),
+    period:     str            = Query("recent", description="recent|today|tomorrow|yesterday|this_week|next_week|previous_week|this_month|next_month|previous_month|custom"),
+    date_from:  Optional[str] = Query(None, description="YYYY-MM-DD (used when period=custom)"),
+    date_to:    Optional[str] = Query(None, description="YYYY-MM-DD (used when period=custom)"),
     currencies: Optional[str] = Query(None, description="Comma-separated: USD,EUR,GBP,..."),
     impacts:    Optional[str] = Query(None, description="Comma-separated: high,medium,low"),
-    limit:      int            = Query(300, ge=1, le=2000),
+    limit:      int            = Query(500, ge=1, le=5000),
     offset:     int            = Query(0, ge=0),
 ) -> Dict[str, Any]:
     """Unified Forex Factory calendar — past releases + upcoming events."""
@@ -433,6 +435,8 @@ def ff_calendar(
     imp_list = [i.strip().lower() for i in impacts.split(",") if i.strip()] if impacts else None
     return get_calendar(
         period=period,
+        date_from=date_from,
+        date_to=date_to,
         currencies=cur_list,
         impacts=imp_list,
         limit=limit,