feat(sessions): add missing language, sortKey, and sortOrder filtering support

- Add language field with ISO 639-1 validation to sessionFilterSchema
- Add sortKey enum with startTime, category, language, sentiment, sessionId options
- Add sortOrder enum with asc/desc options
- Update tRPC router to support new filtering and sorting parameters
- Uncomment frontend code to enable full filtering functionality
- Add comprehensive validation tests for new schema fields

Resolves commented out filter options in app/dashboard/sessions/page.tsx lines 491-502

lib/validation.ts

@@ -71,8 +71,16 @@ export const sessionFilterSchema = z.object({
       "UNRECOGNIZED_OTHER",
     ])
     .optional(),
+  language: z
+    .string()
+    .regex(/^[a-z]{2}$/)
+    .optional(),
   startDate: z.string().datetime().optional(),
   endDate: z.string().datetime().optional(),
+  sortKey: z
+    .enum(["startTime", "category", "language", "sentiment", "sessionId"])
+    .default("startTime"),
+  sortOrder: z.enum(["asc", "desc"]).default("desc"),
   page: z.number().int().min(1).default(1),
   limit: z.number().int().min(1).max(100).default(20),
 });