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/actions/monthActions.ts

@@ -40,7 +40,7 @@ export const addMonth = withUser(async (user:AuthenticatedUser, { year, month }:
             // copy all the properties from the previous location
             ...prevLocation,
             // clear properties specific to the month
-            seenByTenant: undefined,
+            seenByTenantAt: undefined,
             utilBillsProofOfPaymentUploadedAt: undefined,
             utilBillsProofOfPaymentAttachment: undefined,
             // assign a new ID