feat: implement filter logic with reactive KPI updates (Task 3.3)

- Add apply_filters() method that queries SQLite with current filter state
- Handle initiated date filter (first intervention date range)
- Handle last seen date filter (last intervention date range)
- Handle drug and directorate multi-select filters
- Use CTE pattern for efficient patient-level date filtering
- Update KPI values (unique_patients, total_drugs, total_cost) on filter change
- Call apply_filters() from all filter event handlers
- Call apply_filters() after initial data load

IMPLEMENTATION_PLAN.md

@@ -122,11 +122,11 @@ cd pathways_app && timeout 60 python -m reflex run 2>&1 | head -30
 - [x] Call on app initialization
 
 ### 3.3 Filter Logic
-- [ ] Create `apply_filters()` computed method that filters the data based on current state
-- [ ] Handle initiated date filter (when enabled)
-- [ ] Handle last seen date filter (when enabled)
-- [ ] Handle drug/indication/directorate multi-select filters
-- [ ] Return filtered DataFrame
+- [x] Create `apply_filters()` computed method that filters the data based on current state
+- [x] Handle initiated date filter (when enabled)
+- [x] Handle last seen date filter (when enabled)
+- [x] Handle drug/indication/directorate multi-select filters
+- [x] Return filtered DataFrame
 
 ### 3.4 KPI Calculations
 - [ ] Create computed properties for KPI values: