feat(proxies): CM-9 backend — polymorphic point-of-contact + resolver

- proxies table (migration 0095, port_id cascade), one per client/interest/yacht
- service: get/set(upsert)/clear + resolveEffectiveProxy (yacht → interest →
  client precedence), port-scoped with entity-in-port guard
- per-entity sub-resource routes (/clients|interests|yachts/[id]/proxy) reusing
  each entity's existing view/edit permission (no new permission resource)
- 3 integration tests (CRUD/upsert, tenant guard, resolution precedence)

Backend only — ProxyCard UI on the 3 detail pages to follow. tsc clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lib/api/proxy-route-handlers.ts

@@ -0,0 +1,56 @@
+/**
+ * CM-9: shared GET/PUT/DELETE handlers for the per-entity proxy sub-resource
+ * (`/api/v1/{clients|interests|yachts}/[id]/proxy`). Each entity's route.ts
+ * binds these with its own permission resource so we reuse existing
+ * clients/interests/yachts gating instead of a new permission.
+ */
+
+import { NextResponse } from 'next/server';
+
+import { type RouteHandler } from '@/lib/api/helpers';
+import { parseBody } from '@/lib/api/route-helpers';
+import { errorResponse } from '@/lib/errors';
+import { clearProxy, getProxy, setProxy } from '@/lib/services/proxies.service';
+import { setProxySchema, type ProxyEntityType } from '@/lib/validators/proxies';
+
+export function makeProxyHandlers(entityType: ProxyEntityType) {
+  const getHandler: RouteHandler = async (req, ctx, params) => {
+    try {
+      const proxy = await getProxy(ctx.portId, entityType, params.id!);
+      return NextResponse.json({ data: proxy });
+    } catch (error) {
+      return errorResponse(error);
+    }
+  };
+
+  const putHandler: RouteHandler = async (req, ctx, params) => {
+    try {
+      const body = await parseBody(req, setProxySchema);
+      const proxy = await setProxy(ctx.portId, entityType, params.id!, body, {
+        userId: ctx.userId,
+        portId: ctx.portId,
+        ipAddress: ctx.ipAddress,
+        userAgent: ctx.userAgent,
+      });
+      return NextResponse.json({ data: proxy });
+    } catch (error) {
+      return errorResponse(error);
+    }
+  };
+
+  const deleteHandler: RouteHandler = async (req, ctx, params) => {
+    try {
+      await clearProxy(ctx.portId, entityType, params.id!, {
+        userId: ctx.userId,
+        portId: ctx.portId,
+        ipAddress: ctx.ipAddress,
+        userAgent: ctx.userAgent,
+      });
+      return new NextResponse(null, { status: 204 });
+    } catch (error) {
+      return errorResponse(error);
+    }
+  };
+
+  return { getHandler, putHandler, deleteHandler };
+}