fix: address CodeRabbit review on PR #1569

Critical:
- migrations: change version 8 → 25 to avoid collision with
  MigrationRunner.addObservationHierarchicalFields (uses version 8)
- SessionRoutes: remove duplicate imports that prevent compilation

Major:
- SessionRoutes: call applyTierRouting() before every generator spawn
  (stale-recovery and crash-recovery paths were missing it)
- applyTierRouting: clear session.modelOverride at top before re-evaluating
  to prevent stale tier from persisting across spawns

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/services/sqlite/migrations.ts

@@ -519,7 +519,7 @@ export const migration007: Migration = {
  * explicit retrieval). Foundation for future Thompson Sampling optimization.
  */
 export const migration008: Migration = {
-  version: 8,
+  version: 25,
   up: (db: Database) => {
     db.run(`
       CREATE TABLE IF NOT EXISTS observation_feedback (