fix(tests): fast vector-sync cadence for multi-user-basic CI service

After restoring the background-sync UI ids, the two indexing-dependent
multi-user-basic tests (chunk_context_uses_app_password, plotly_with_basic_auth)
surfaced a second, previously-masked failure: they provision a user, create a
note, then wait ~90s for it to be indexed.

The mcp-multi-user-basic service ran with the production cadence — scan
interval 60s and the default user-poll interval 60s. A freshly-provisioned
user isn't even *discovered* by the background-sync user manager for up to
60s, leaving too little of the 90s budget for the scan + single-worker
indexing to finish (observed: pending docs still "syncing" at timeout, or the
scanner not yet started → "idle" with 0 indexed).

Match the single-user service's short cadence (5s) and add a matching 5s
user-poll interval so discovery + scan + index complete well within the test
budget. Test-only config; production deployments set their own intervals.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -160,7 +160,15 @@ services:
       - TOKEN_STORAGE_DB=/app/data/tokens.db
 
       - ENABLE_SEMANTIC_SEARCH=true
-      - VECTOR_SYNC_SCAN_INTERVAL=60
+      # Fast discovery + scan cadence for integration tests: the multi-user
+      # background-sync suite provisions a user, creates a note, then waits
+      # ~90s for it to be indexed. With the production defaults (user-poll 60s
+      # + scan 60s) a freshly-provisioned user often isn't even discovered
+      # within that budget, so the wait times out (note never indexed). Match
+      # the single-user service's short cadence so discovery + scan + index
+      # complete promptly under CI load.
+      - VECTOR_SYNC_USER_POLL_INTERVAL=5
+      - VECTOR_SYNC_SCAN_INTERVAL=5
       - VECTOR_SYNC_PROCESSOR_WORKERS=1
 
       # OAuth credentials for background sync (optional - uses DCR if not provided)