feat(mvp): phase 8 - integration, testing & deployment

Fix all build/type/lint errors (zod 3.25 compat wrapper, Svelte 5 fixes),
write 115 unit tests across 10 test files, expand seed script with demo
data, update Docker config with migration on startup.

src/lib/components/admin/SettingsForm.svelte

@@ -136,7 +136,7 @@
 				bind:value={$form.healthcheckDefaults}
 				rows="4"
 				class="w-full rounded-md border border-input bg-background px-3 py-2 font-mono text-sm text-foreground"
-				placeholder='{"interval": 300, "timeout": 5000, "method": "GET"}'
+				placeholder={'{"interval": 300, "timeout": 5000, "method": "GET"}'}
 			></textarea>
 			{#if $errors.healthcheckDefaults}<span class="text-xs text-destructive">{$errors.healthcheckDefaults}</span>{/if}
 		</div>