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/database/queries/boundarylogs.sql

@@ -39,7 +39,7 @@ SELECT
     unnest(@proto :: text[]),
     unnest(@method :: text[]),
     unnest(@detail :: text[]),
-    unnest(@matched_rule :: text[])
+    NULLIF(unnest(@matched_rule :: text[]), '')
 RETURNING *;
 
 -- name: GetBoundaryLogByID :one