perf: batch processor and log inserts to reduce ClickHouse part creation

Diagnostics showed ~3,200 tiny inserts per 5 minutes:
- processor_executions: 2,376 inserts (14 rows avg) — one per chunk
- logs: 803 inserts (5 rows avg) — synchronous in HTTP handler

Fix 1: Consolidate processor inserts — new insertProcessorBatches() method
flattens all ProcessorBatch records into a single INSERT per flush cycle.

Fix 2: Buffer log inserts — route through WriteBuffer<BufferedLogEntry>,
flushed on the same 5s interval as executions. LogIngestionController now
pushes to buffer instead of inserting directly.

Also reverts async_insert config (doesn't work with JDBC inline VALUES).

Expected: ~3,200 inserts/5min → ~160 (20x reduction in part creation,
MV triggers, and background merge work).

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

cameleer3-server-core/src/main/java/com/cameleer3/server/core/ingestion/BufferedLogEntry.java

@@ -0,0 +1,12 @@
+package com.cameleer3.server.core.ingestion;
+
+import com.cameleer3.common.model.LogEntry;
+
+/**
+ * A log entry paired with its agent metadata, ready for buffered ClickHouse insertion.
+ */
+public record BufferedLogEntry(
+        String instanceId,
+        String applicationId,
+        LogEntry entry
+) {}