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.

pages/8_Export.py

@@ -5,7 +5,7 @@ from auth import require_auth
 from nightscout_client import NightscoutConfigError
 from pdf_export import AVAILABLE_SECTIONS, SECTION_EVENTS, build_report_pdf
 from reporting import HIGH, LOW, VERY_HIGH, VERY_LOW, detect_events, summarize_events
-from ui_common import load_range_data, select_period_days
+from ui_common import load_range_data, render_completeness_warning, select_period_days
 
 st.set_page_config(page_title="Glucose dashboard — Export", page_icon="📊", layout="wide")
 require_auth()
@@ -29,6 +29,8 @@ if glucose.empty:
     st.info("Geen glucosemetingen gevonden in deze periode.")
     st.stop()
 
+render_completeness_warning(glucose, start, end)
+
 events_summary = {}
 if SECTION_EVENTS in sections:
     levels = {