fix: Nightscout API silently truncated multi-day fetches to ~3-4 days

_entries_since/_treatments_since requested a single page via `count`,
but Nightscout's /api/v1/entries and /api/v1/treatments ignore a large
`count` and cap the response around ~1150/~700 documents regardless of
how far back the requested period goes. Every report page relying on
fetch_entries_range/fetch_treatments_range (7/14/30/90-day selectors)
was silently only getting the most recent ~3-4 days, no matter the
selected period.

Fixed by paginating with `find[date][$lte]` cursors in a new
_paginated_get(), walking back until the requested `since` boundary is
reached or the server returns a short/empty page (= genuinely no more
data, confirmed by manually paginating against the real instance:
entries go back 50+ days, treatments only ~4 days on this install).

Also added reporting.completeness_warning(), shared by the interactive
pages (via ui_common.render_completeness_warning) and the PDF export,
that distinguishes "a structural gap at the start of the period" (data
wasn't fetched/available) from "scattered gaps below the clinical AGP
coverage threshold" (Danne et al. 2017) — the previous single generic
"X% sensor coverage" message read as a CGM/sensor problem even when
the real cause was this fetch bug.

ui_common.py

@@ -15,7 +15,7 @@ import plotly.graph_objects as go
 import streamlit as st
 
 from nightscout_client import fetch_entries_range, fetch_treatments_range
-from reporting import detect_events, summarize_events
+from reporting import completeness_warning, detect_events, summarize_events
 
 MMOL_HOVERTEMPLATE = "%{x|%d-%m %H:%M} · %{y:.1f} mmol/L<extra></extra>"
 
@@ -98,6 +98,18 @@ def load_range_data(start: date, end: date):
     return glucose, treatments
 
 
+def render_completeness_warning(glucose: pd.DataFrame, start: date, end: date) -> None:
+    """Toon reporting.completeness_warning()'s oordeel als st.warning, indien nodig.
+
+    Gebruikt door elke rapportagepagina, direct na de load_range_data-call.
+    De beoordelingslogica zelf staat in reporting.py (pure functie) zodat het
+    PDF-rapport (pdf_export.py) exact dezelfde disclaimer kan hergebruiken.
+    """
+    message = completeness_warning(glucose, start, end)
+    if message:
+        st.warning(message)
+
+
 def render_event_level(
     glucose: pd.DataFrame,
     threshold: float,