feat: add UI usage analytics tracking

Tracks authenticated UI user requests to understand usage patterns:
- New ClickHouse usage_events table with 90-day TTL
- UsageTrackingInterceptor captures method, path, duration, user
- Path normalization groups dynamic segments ({id}, {hash})
- Buffered writes via WriteBuffer + periodic flush
- Admin endpoint GET /api/v1/admin/usage with groupBy=endpoint|user|hour
- Skips agent requests, health checks, and data ingestion

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cameleer3-server-core/src/main/java/com/cameleer3/server/core/analytics/UsageEvent.java

@@ -0,0 +1,14 @@
+package com.cameleer3.server.core.analytics;
+
+import java.time.Instant;
+
+public record UsageEvent(
+        Instant timestamp,
+        String username,
+        String method,
+        String path,
+        String normalized,
+        int statusCode,
+        long durationMs,
+        String queryParams
+) {}

cameleer3-server-core/src/main/java/com/cameleer3/server/core/analytics/UsageStats.java

@@ -0,0 +1,7 @@
+package com.cameleer3.server.core.analytics;
+
+public record UsageStats(
+        String key,
+        long count,
+        long avgDurationMs
+) {}

cameleer3-server-core/src/main/java/com/cameleer3/server/core/analytics/UsageTracker.java

@@ -0,0 +1,6 @@
+package com.cameleer3.server.core.analytics;
+
+public interface UsageTracker {
+
+    void track(UsageEvent event);
+}