feat: add partial unique index for single primary address per client

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/lib/db/migrations/0001_soft_ender_wiggin.sql

@@ -0,0 +1 @@
+CREATE UNIQUE INDEX "idx_ca_primary" ON "client_addresses" USING btree ("client_id") WHERE "client_addresses"."is_primary" = true;

src/lib/db/migrations/meta/_journal.json

@@ -8,6 +8,13 @@
       "when": 1776185027494,
       "tag": "0000_narrow_longshot",
       "breakpoints": true
+    },
+    {
+      "idx": 1,
+      "version": "7",
+      "when": 1776185487775,
+      "tag": "0001_soft_ender_wiggin",
+      "breakpoints": true
     }
   ]
 }