add dashboard filtering to analytics

Click any row in pages, sources, countries, or devices tables to
filter the entire dashboard by that dimension. Active filters show
as dismissible chips. Filters thread through all queries including
previous-period deltas. 1050 tests.

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

test/berrypod/analytics_test.exs

@@ -231,6 +231,62 @@ defmodule Berrypod.AnalyticsTest do
     end
   end
 
+  describe "filtering" do
+    test "count_visitors respects country_code filter" do
+      v1 = :crypto.strong_rand_bytes(8)
+      v2 = :crypto.strong_rand_bytes(8)
+
+      insert_event(%{visitor_hash: v1, country_code: "GB"})
+      insert_event(%{visitor_hash: v2, country_code: "US"})
+
+      assert Analytics.count_visitors(today_range(), %{country_code: "GB"}) == 1
+    end
+
+    test "top_pages respects referrer_source filter" do
+      v1 = :crypto.strong_rand_bytes(8)
+      v2 = :crypto.strong_rand_bytes(8)
+
+      insert_event(%{visitor_hash: v1, pathname: "/", referrer_source: "Google"})
+      insert_event(%{visitor_hash: v2, pathname: "/about", referrer_source: "Facebook"})
+
+      pages = Analytics.top_pages(today_range(), filters: %{referrer_source: "Google"})
+      assert length(pages) == 1
+      assert hd(pages).pathname == "/"
+    end
+
+    test "bounce_rate respects filter" do
+      s1 = :crypto.strong_rand_bytes(8)
+      s2 = :crypto.strong_rand_bytes(8)
+
+      # GB visitor bounces (1 pageview)
+      insert_event(%{session_hash: s1, country_code: "GB"})
+      # US visitor doesn't bounce (2 pageviews)
+      insert_event(%{session_hash: s2, country_code: "US", pathname: "/"})
+      insert_event(%{session_hash: s2, country_code: "US", pathname: "/about"})
+
+      assert Analytics.bounce_rate(today_range(), %{country_code: "GB"}) == 100
+      assert Analytics.bounce_rate(today_range(), %{country_code: "US"}) == 0
+    end
+
+    test "multiple filters combine with AND logic" do
+      v1 = :crypto.strong_rand_bytes(8)
+      v2 = :crypto.strong_rand_bytes(8)
+
+      insert_event(%{visitor_hash: v1, country_code: "GB", browser: "Chrome"})
+      insert_event(%{visitor_hash: v2, country_code: "GB", browser: "Firefox"})
+
+      assert Analytics.count_visitors(today_range(), %{country_code: "GB", browser: "Chrome"}) ==
+               1
+    end
+
+    test "unknown filter keys are ignored" do
+      v1 = :crypto.strong_rand_bytes(8)
+      insert_event(%{visitor_hash: v1})
+
+      assert Analytics.count_visitors(today_range(), %{nonexistent: "val"}) == 1
+    end
+  end
+
   describe "delete_events_before/1" do
     test "deletes old events" do
       old = DateTime.add(DateTime.utc_now(), -400, :day) |> DateTime.truncate(:second)