Add click heatmaps report

Extracts click events from rrweb session-replay buffers at write time
into a new heatmap_event table (Prisma + ClickHouse). New /heatmaps
report shows a page picker plus an overlay of click density on the
dominant viewport, with an optional iframe of the live page behind.

db/clickhouse/migrations/12_add_heatmap.sql

@@ -0,0 +1,21 @@
+-- Create heatmap_event
+CREATE TABLE umami.heatmap_event
+(
+    heatmap_event_id UUID,
+    website_id UUID,
+    session_id UUID,
+    visit_id UUID,
+    url_path String,
+    event_type UInt8,
+    node_id Nullable(Int32),
+    x Nullable(Int32),
+    y Nullable(Int32),
+    viewport_w Nullable(Int32),
+    viewport_h Nullable(Int32),
+    scroll_pct Nullable(UInt8),
+    created_at DateTime('UTC')
+)
+ENGINE = MergeTree
+    PARTITION BY toYYYYMM(created_at)
+    ORDER BY (website_id, url_path, event_type, created_at)
+    SETTINGS index_granularity = 8192;

db/clickhouse/schema.sql

@@ -344,3 +344,25 @@ AS SELECT
     groupArrayState(data_type) AS property_types
 FROM umami.event_data
 GROUP BY website_id, session_id, event_id, event_name, url_path, created_at;
+
+-- Create heatmap_event
+CREATE TABLE umami.heatmap_event
+(
+    heatmap_event_id UUID,
+    website_id UUID,
+    session_id UUID,
+    visit_id UUID,
+    url_path String,
+    event_type UInt8,
+    node_id Nullable(Int32),
+    x Nullable(Int32),
+    y Nullable(Int32),
+    viewport_w Nullable(Int32),
+    viewport_h Nullable(Int32),
+    scroll_pct Nullable(UInt8),
+    created_at DateTime('UTC')
+)
+ENGINE = MergeTree
+    PARTITION BY toYYYYMM(created_at)
+    ORDER BY (website_id, url_path, event_type, created_at)
+    SETTINGS index_granularity = 8192;