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/migrations/000520_drop_boundary_logs_session_fk.down.sql

@@ -0,0 +1,10 @@
+-- Delete orphaned logs that have no matching session before restoring
+-- the FK constraint.
+DELETE FROM boundary_logs bl
+WHERE NOT EXISTS (
+    SELECT 1 FROM boundary_sessions bs WHERE bs.id = bl.session_id
+);
+
+ALTER TABLE boundary_logs
+    ADD CONSTRAINT boundary_logs_session_id_fkey
+    FOREIGN KEY (session_id) REFERENCES boundary_sessions(id) ON DELETE CASCADE;

coderd/database/migrations/000520_drop_boundary_logs_session_fk.up.sql

@@ -0,0 +1,6 @@
+-- Drop the foreign key so that boundary logs can be inserted before
+-- the session row exists. The session is created lazily and may fail
+-- on transient errors; removing the FK lets logs persist regardless.
+-- The session row will be created on a subsequent batch, retroactively
+-- linking the orphaned logs via session_id.
+ALTER TABLE boundary_logs DROP CONSTRAINT boundary_logs_session_id_fkey;