feat: route PHP Sentry events to Spotlight in dev

Add a when@dev block to sentry.yaml that initialises the PHP Sentry SDK
with a placeholder DSN and forwards all events to the Spotlight sidecar
over the internal Docker network (http://spotlight:8969/stream).

The JS SDK already uses localhost:8969 via spotlight:true when no real
SENTRY_DSN is set, which is correct since the browser cannot reach the
internal Docker hostname.

config/packages/sentry.yaml

@@ -1,3 +1,9 @@
+when@dev:
+    sentry:
+        dsn: 'https://placeholder@placeholder.ingest.sentry.io/0'
+        options:
+            spotlight: 'http://spotlight:8969/stream'
+
 when@prod:
     sentry:
         dsn: '%env(SENTRY_DSN)%'