fix(analysis): _expected_ticks usa ceiling division (no off-by-one)

Il piano originale aveva `floor(span/15) + 1` che over-conta a span allineati
(span=60min → 5 invece di 4). Il primo fix dell'implementer (`floor(span/15)`)
under-conta a span non-allineati (span=16min → 1 invece di 2). Solo
`ceil(span/15)` è corretto in entrambi i casi. Aggiunti 2 test che
coprono gli scenari non-allineato e boundary-esatto per impedire
regressioni. Plan doc allineato.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/test_data_audit.py

@@ -26,3 +26,20 @@ def test_expected_ticks_unaligned_since_rounds_up() -> None:
     until = datetime(2026, 5, 12, 12, 30, tzinfo=UTC)
     # First aligned tick after 12:07 is 12:15, then 12:30 is excluded
     assert _expected_ticks(since, until) == 1
+
+
+def test_expected_ticks_non_multiple_span_uses_ceiling() -> None:
+    # Span = 16 min (12:00 → 12:16): ticks 12:00 AND 12:15 are < 12:16.
+    # floor(16/15) = 1 would under-count; the correct answer is 2.
+    since = datetime(2026, 5, 12, 12, 0, tzinfo=UTC)
+    until = datetime(2026, 5, 12, 12, 16, tzinfo=UTC)
+    assert _expected_ticks(since, until) == 2
+
+
+def test_expected_ticks_exact_quarter_boundary_is_excluded() -> None:
+    # Until landing exactly on a */15 tick: that tick is NOT counted
+    # because the window is half-open on the right.
+    since = datetime(2026, 5, 12, 12, 0, tzinfo=UTC)
+    until = datetime(2026, 5, 12, 12, 45, tzinfo=UTC)
+    # Ticks at 12:00, 12:15, 12:30 → 3 (12:45 excluded)
+    assert _expected_ticks(since, until) == 3