Replace seenByTenant boolean with seenByTenantAt timestamp field

Update location tracking to record when tenant views a location rather than just whether they've seen it. This provides better audit trail and enables future features like viewing history.

Changes:
- Convert seenByTenant (boolean) to seenByTenantAt (Date) in database schema
- Update setSeenByTenantAt action to store timestamp instead of boolean flag
- Modify LocationCard UI to display when location was seen by tenant
- Update all references across locationActions, monthActions, and view components
- Remove unused imports from ViewLocationCard

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/lib/db-types.ts

@@ -74,7 +74,7 @@ export interface BillingLocation {
     /** (optional) monthly rent amount in cents */
     rentAmount?: number | null;
     /** (optional) whether the location has been seen by tenant */
-    seenByTenant?: boolean | null;
+    seenByTenantAt?: Date | null;
     /** (optional) utility bills proof of payment attachment */
     utilBillsProofOfPaymentAttachment?: BillAttachment|null;
     /** (optional) date when utility bills proof of payment was uploaded */