fix: add pagination to unbounded GET endpoints

All list endpoints now accept ?page and ?limit (default 50, max 200) and
return { data, total, page, limit } instead of a bare array, preventing
memory and performance failures at scale.

- GET /api/dogs: adds pagination, server-side search (?search) and sex
  filter (?sex), and a stats aggregate (total/males/females) for the
  Dashboard to avoid counting from the array
- GET /api/litters: adds pagination; also fixes N+1 query by fetching
  all puppies for the current page in a single query instead of one per
  litter
- DogList: moves search/sex filtering server-side with 300ms debounce;
  adds Prev/Next pagination controls
- LitterList: uses paginated response; adds Prev/Next pagination controls
- Dashboard: reads counts from stats/total fields instead of array length
- LitterDetail, LitterForm: switch dogs fetch to /api/dogs/all (complete
  list, no pagination, for sire/dam dropdowns)
- DogForm: updates litters fetch to use paginated response shape

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/components/DogForm.jsx

@@ -64,8 +64,8 @@ function DogForm({ dog, onClose, onSave, isExternal = false }) {
 
   const fetchLitters = async () => {
     try {
-      const res = await axios.get('/api/litters')
-      const data = res.data || []
+      const res = await axios.get('/api/litters', { params: { limit: 200 } })
+      const data = res.data.data || []
       setLitters(data)
       setLittersAvailable(data.length > 0)
       if (data.length === 0) setUseManualParents(true)

client/src/components/LitterForm.jsx

@@ -39,7 +39,7 @@ function LitterForm({ litter, prefill, onClose, onSave }) {
 
   const fetchDogs = async () => {
     try {
-      const res = await axios.get('/api/dogs')
+      const res = await axios.get('/api/dogs/all')
       setDogs(res.data)
     } catch (error) {
       console.error('Error fetching dogs:', error)