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/3_AGP_Profiel.py

@@ -6,7 +6,13 @@ from auth import require_auth
 from data_processing import TARGET_HIGH, TARGET_LOW
 from nightscout_client import NightscoutConfigError
 from reporting import agp_percentile_bands
-from ui_common import get_colors, hex_to_rgba, load_range_data, select_period_days
+from ui_common import (
+    get_colors,
+    hex_to_rgba,
+    load_range_data,
+    render_completeness_warning,
+    select_period_days,
+)
 
 st.set_page_config(page_title="Glucose dashboard — AGP profiel", page_icon="📊", layout="wide")
 require_auth()
@@ -32,6 +38,8 @@ if glucose.empty:
     st.info("Geen glucosemetingen gevonden in deze periode.")
     st.stop()
 
+render_completeness_warning(glucose, start, end)
+
 bands = agp_percentile_bands(glucose, bin_minutes=30)
 colors = get_colors()