feat: add Zakat calculator (per-member, Nisab + 2.5% rate)

Closes the Zakat gap identified in the competitive benchmark against
Rafiq. Per-member module (nf_zakat_records, same author-owns/family-reads
RLS pattern as Insurance/Wassiyah/Waqf) computing 2.5% due on zakatable
wealth (cash, gold, silver, business assets, investments) once above a
user-entered Nisab threshold, minus deductible short-term liabilities.
Manual entry only, matching the app's existing philosophy — no live gold
price feed. Covered by e2e-zakat.cjs (7/7).

src/lib/db.js

@@ -490,3 +490,25 @@ export async function setAssetVerified(assetId, verifiedBy, verified) {
   }).eq('id', assetId);
   if (error) throw error;
 }
+
+// ── Zakat — per-member, one live record per family/member pair. ──
+export async function getZakatRecord(familyId, memberId) {
+  const { data, error } = await supabase.from('nf_zakat_records').select('*').eq('family_id', familyId).eq('member_id', memberId).maybeSingle();
+  if (error) throw error;
+  if (!data) return null;
+  return {
+    cash: data.cash, gold: data.gold, silver: data.silver, businessAssets: data.business_assets,
+    investments: data.investments, otherZakatable: data.other_zakatable,
+    deductibleLiabilities: data.deductible_liabilities, nisabThreshold: data.nisab_threshold
+  };
+}
+export async function upsertZakatRecord(familyId, memberId, fields) {
+  const { error } = await supabase.from('nf_zakat_records').upsert({
+    family_id: familyId, member_id: memberId,
+    cash: Number(fields.cash) || 0, gold: Number(fields.gold) || 0, silver: Number(fields.silver) || 0,
+    business_assets: Number(fields.businessAssets) || 0, investments: Number(fields.investments) || 0,
+    other_zakatable: Number(fields.otherZakatable) || 0, deductible_liabilities: Number(fields.deductibleLiabilities) || 0,
+    nisab_threshold: Number(fields.nisabThreshold) || 0, updated_at: new Date().toISOString()
+  }, { onConflict: 'family_id,member_id' });
+  if (error) throw error;
+}