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.

pdf_export.py

@@ -22,6 +22,7 @@ from fpdf.enums import XPos, YPos
 
 from reporting import (
     agp_percentile_bands,
+    completeness_warning,
     gmi_percent,
     percent_cv,
     sensor_active_percent,
@@ -64,6 +65,13 @@ def _add_summary_section(pdf: FPDF, glucose: pd.DataFrame, start, end) -> None:
 
     pdf.set_font("Helvetica", "B", 13)
     pdf.cell(0, 8, "Samenvatting", **_LINE_KWARGS)
+
+    warning = completeness_warning(glucose, start, end)
+    if warning:
+        pdf.set_font("Helvetica", "BI", 10)
+        pdf.multi_cell(0, 5, warning)
+        pdf.ln(2)
+
     pdf.set_font("Helvetica", "", 11)
     pdf.cell(0, 6, f"Sensordekking: {active_pct:.0f}%", **_LINE_KWARGS)
     pdf.cell(0, 6, f"Gemiddelde glucose: {mean_mmol:.1f} mmol/L", **_LINE_KWARGS)