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>

server/routes/litters.js

@@ -2,31 +2,50 @@ const express = require('express');
 const router = express.Router();
 const { getDatabase } = require('../db/init');
 
-// GET all litters
+// GET all litters (paginated)
+// ?page=1&limit=50
+// Response: { data, total, page, limit }
 router.get('/', (req, res) => {
   try {
     const db = getDatabase();
+    const page  = Math.max(1, parseInt(req.query.page, 10) || 1);
+    const limit = Math.min(200, Math.max(1, parseInt(req.query.limit, 10) || 50));
+    const offset = (page - 1) * limit;
+
+    const total = db.prepare('SELECT COUNT(*) as count FROM litters').get().count;
+
     const litters = db.prepare(`
-      SELECT l.*, 
+      SELECT l.*,
         s.name as sire_name, s.registration_number as sire_reg,
         d.name as dam_name, d.registration_number as dam_reg
       FROM litters l
       JOIN dogs s ON l.sire_id = s.id
       JOIN dogs d ON l.dam_id = d.id
       ORDER BY l.breeding_date DESC
-    `).all();
-    
-    litters.forEach(litter => {
-      litter.puppies = db.prepare(`
-        SELECT * FROM dogs WHERE litter_id = ? AND is_active = 1
-      `).all(litter.id);
-      litter.puppies.forEach(puppy => {
-        puppy.photo_urls = puppy.photo_urls ? JSON.parse(puppy.photo_urls) : [];
+      LIMIT ? OFFSET ?
+    `).all(limit, offset);
+
+    if (litters.length > 0) {
+      const litterIds = litters.map(l => l.id);
+      const placeholders = litterIds.map(() => '?').join(',');
+      const allPuppies = db.prepare(`
+        SELECT * FROM dogs WHERE litter_id IN (${placeholders}) AND is_active = 1
+      `).all(...litterIds);
+
+      const puppiesByLitter = {};
+      allPuppies.forEach(p => {
+        p.photo_urls = p.photo_urls ? JSON.parse(p.photo_urls) : [];
+        if (!puppiesByLitter[p.litter_id]) puppiesByLitter[p.litter_id] = [];
+        puppiesByLitter[p.litter_id].push(p);
       });
-      litter.actual_puppy_count = litter.puppies.length;
-    });
-    
-    res.json(litters);
+
+      litters.forEach(l => {
+        l.puppies = puppiesByLitter[l.id] || [];
+        l.actual_puppy_count = l.puppies.length;
+      });
+    }
+
+    res.json({ data: litters, total, page, limit });
   } catch (error) {
     res.status(500).json({ error: error.message });
   }