feat(webapp): add EVENTS_READER_CLICKHOUSE_URL for events read replica

The events client both inserts events and reads traces/spans/logs through one
instance. Give it a dedicated reader URL so reads can move to a replica while
inserts stay on EVENTS_CLICKHOUSE_URL. Unlike the read-only clients, it does
not fall back to CLICKHOUSE_READER_URL: a write-capable client only moves reads
on an explicit opt-in.

Author: nicktrn

.server-changes/events-reader-clickhouse-url.md

@@ -0,0 +1,6 @@
+---
+area: webapp
+type: improvement
+---
+
+Add `EVENTS_READER_CLICKHOUSE_URL` to send trace/span/log reads to a read replica while event inserts stay on `EVENTS_CLICKHOUSE_URL`. Optional; unset keeps reads and writes on the same instance.

apps/webapp/app/env.server.ts

@@ -1816,6 +1816,8 @@ const EnvironmentSchema = z
       .string()
       .optional()
       .transform((v) => v ?? process.env.CLICKHOUSE_URL),
+    // Events read replica (traces/spans/logs). No CLICKHOUSE_READER_URL fallback by design: this write-capable client opts in explicitly.
+    EVENTS_READER_CLICKHOUSE_URL: z.string().optional(),
     EVENTS_CLICKHOUSE_KEEP_ALIVE_ENABLED: z.string().default("1"),
     EVENTS_CLICKHOUSE_KEEP_ALIVE_IDLE_SOCKET_TTL_MS: z.coerce.number().int().optional(),
     EVENTS_CLICKHOUSE_MAX_OPEN_CONNECTIONS: z.coerce.number().int().default(10),

apps/webapp/app/services/clickhouse/clickhouseFactory.server.ts

@@ -307,10 +307,9 @@ function initializeEventsClickhouseClient(): ClickHouse {
     maxOpenConnections: env.EVENTS_CLICKHOUSE_MAX_OPEN_CONNECTIONS,
   };
 
-  // This client both inserts events and reads traces/spans/logs. When a reader replica
-  // is configured, split it so queries hit the replica while inserts stay on the writer.
-  if (env.CLICKHOUSE_READER_URL) {
-    const readerUrl = new URL(env.CLICKHOUSE_READER_URL);
+  // Mixed read+write client: split reads to its own EVENTS_READER_CLICKHOUSE_URL (not the global reader) so inserts can never hit the replica.
+  if (env.EVENTS_READER_CLICKHOUSE_URL) {
+    const readerUrl = new URL(env.EVENTS_READER_CLICKHOUSE_URL);
     readerUrl.searchParams.delete("secure");
 
     if (readerUrl.toString() !== writerUrl.toString()) {