perf(database): scope listSnapshot table reads to current company

Add explicit LIMIT to all hot-path tables in listSnapshot regardless of
companyId, raise cross-company caps from 500/250 to 2000/1000 so dashboard
no longer truncates silently at 500.

Add migration 28 with composite indexes covering snapshot WHERE/ORDER BY
columns: tasks(company_id, created_at|updated_at), approvals(company_id,
created_at), task_comments(company_id, created_at), documents(company_id,
updated_at), meetings(company_id, scheduled_at), sprints(company_id,
start_date), runs(agent_id, created_at), cost_entries(agent_id,
created_at), activity(company_id, actor, action), tasks(company_id,
assignee_agent_id, status). Indexes match exact ORDER BY direction so
SQLite can satisfy reads via index scan without temp sort.

src/main/database/schema.ts

@@ -407,6 +407,20 @@ export function runMigrations(
         }
       },
     },
+    {
+      version: 28, name: "add_snapshot_hot_path_indexes",
+      sql: `CREATE INDEX IF NOT EXISTS idx_tasks_company_created_at ON tasks(company_id, created_at DESC);
+            CREATE INDEX IF NOT EXISTS idx_tasks_company_updated_at ON tasks(company_id, updated_at DESC);
+            CREATE INDEX IF NOT EXISTS idx_approvals_company_created_at ON approvals(company_id, created_at DESC);
+            CREATE INDEX IF NOT EXISTS idx_task_comments_company_created_at ON task_comments(company_id, created_at);
+            CREATE INDEX IF NOT EXISTS idx_documents_company_updated_at ON documents(company_id, updated_at DESC);
+            CREATE INDEX IF NOT EXISTS idx_meetings_company_scheduled_at ON meetings(company_id, scheduled_at DESC);
+            CREATE INDEX IF NOT EXISTS idx_sprints_company_start_date ON sprints(company_id, start_date DESC);
+            CREATE INDEX IF NOT EXISTS idx_runs_agent_created_at ON runs(agent_id, created_at DESC);
+            CREATE INDEX IF NOT EXISTS idx_cost_entries_agent_created_at ON cost_entries(agent_id, created_at DESC);
+            CREATE INDEX IF NOT EXISTS idx_activity_company_actor_action ON activity(company_id, actor, action);
+            CREATE INDEX IF NOT EXISTS idx_tasks_company_assignee_status ON tasks(company_id, assignee_agent_id, status)`,
+    },
   ];
 
   for (const migration of migrations) {