improve analytics chart with hourly today view and readable labels

- add visitors_by_hour query for hourly breakdown on "today" period
- replace SVG-only chart with HTML/CSS grid layout (bars + labels)
- Y-axis scale with nice rounded max, midpoint, and zero
- X-axis date labels (formatted as "Feb 18") spaced evenly
- adaptive bar gaps (1px for sparse data, 0 for 365-day dense view)
- labels use real HTML text so they're readable on mobile

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/berrypod/analytics.ex

@@ -148,6 +148,29 @@ defmodule Berrypod.Analytics do
     |> Repo.all()
   end
 
+  @doc """
+  Hourly visitor counts for the trend chart (used for "today" period).
+
+  Returns a list of `%{hour: integer, visitors: integer}` maps for all 24 hours.
+  """
+  def visitors_by_hour(date_range) do
+    counts =
+      base_query(date_range)
+      |> where([e], e.name == "pageview")
+      |> group_by([e], fragment("CAST(strftime('%H', ?) AS INTEGER)", e.inserted_at))
+      |> select([e], %{
+        hour: fragment("CAST(strftime('%H', ?) AS INTEGER)", e.inserted_at),
+        visitors: count(e.visitor_hash, :distinct)
+      })
+      |> Repo.all()
+      |> Map.new(&{&1.hour, &1.visitors})
+
+    # Fill in all 24 hours so the chart has no gaps
+    Enum.map(0..23, fn h ->
+      %{hour: h, visitors: Map.get(counts, h, 0)}
+    end)
+  end
+
   @doc """
   Top pages by unique visitors.
   """