perf: eliminate N+1 query patterns across import, vault, and usage pipelines

- Convert importCron/importUserProfiles/importUserOverrides to multi-row
  UPSERT with chunking (200→2 queries per import)
- Add GetTenantsByIDs batch method to TenantStore (PG+SQLite), refactor
  handleMine() to single-query tenant fetch (N→1)
- Refactor buildAgentKeyMap() to use existing GetByKeys with tenant-scoped
  context instead of per-key SELECT (N→1)
- Add GetDocumentsByIDs, GetDocumentByBasename, CreateLinks batch methods
  to VaultStore (PG+SQLite)
- Batch-fetch docs in enrichment Phase 0, carry title through pipeline to
  avoid per-doc refetch in classify phase
- Replace ListDocuments(limit=500) fallback in wikilink resolution with
  targeted GetDocumentByBasename DB query
- Batch CreateLink calls in SyncDocLinks and classifyLinks
- Refactor usage handleGet/handleSummary to use ListPagedRich instead of
  List+GetOrCreate N+1 loop (100→1 queries)
- Batch INSERT for team import members/comments/events/links (250→5)
- Add migration 47: UNIQUE constraint on cron_jobs(agent_id,tenant_id,name)
  with dedup, SQLite schema v15

internal/store/sqlitestore/schema.sql

@@ -482,6 +482,7 @@ CREATE INDEX IF NOT EXISTS idx_cron_jobs_user_id ON cron_jobs(user_id);
 CREATE INDEX IF NOT EXISTS idx_cron_jobs_agent_user ON cron_jobs(agent_id, user_id);
 CREATE INDEX IF NOT EXISTS idx_cron_jobs_team ON cron_jobs(team_id) WHERE team_id IS NOT NULL;
 CREATE INDEX IF NOT EXISTS idx_cron_jobs_tenant ON cron_jobs(tenant_id);
+CREATE UNIQUE INDEX IF NOT EXISTS uq_cron_jobs_agent_tenant_name ON cron_jobs(agent_id, tenant_id, name);
 
 -- ============================================================
 -- Table: cron_run_logs