feat: persist boundary logs (#24812)

Add database persistence to `ReportBoundaryLogs`. On first log for a
session, the handler lazy-creates a `boundary_sessions` row, then
batch-inserts all `BoundaryLog` entries into `boundary_logs`. Structured
logging and usage tracking are preserved. Old boundary clients (no
`session_id`) fall back to log-only mode.

> [!NOTE]
> This PR was authored by Coder Agents.

coderd/agentapi/api.go

@@ -237,6 +237,8 @@ func New(opts Options, workspace database.Workspace, agent database.WorkspaceAge
 
 	api.BoundaryLogsAPI = &BoundaryLogsAPI{
 		Log:                  opts.Log,
+		Database:             opts.Database,
+		AgentID:              opts.AgentID,
 		WorkspaceID:          opts.WorkspaceID,
 		OwnerID:              opts.OwnerID,
 		TemplateID:           workspace.TemplateID,