feat(admin): implement unified submission and newsletter management

Introduce a centralized system to manage all website form submissions and newsletter subscriptions.
- Add `Submission` and `NewsletterSubscription` models with MongoDB schema validation
- Implement `submissionController` and `newsletterSubscriptionController` for CRUD operations and filtering
- Create a unified admin UI for reviewing submissions across different sources (home, request, contact, partnership, newsletter)
- Add database migration scripts for creating collections and indexes
- Refactor partnership inquiry forms to use a fixed field structure
- Update admin navigation and server CORS settings to support PATCH requests

models/submission.js

@@ -0,0 +1,85 @@
+const mongoose = require("mongoose");
+
+const SOURCES = ["home", "request", "contact", "partnership"];
+const STATUSES = ["new", "contacted", "info_provided", "closed"];
+
+const emailPattern = /^\S+@\S+\.\S+$/;
+const phonePattern = /^[+()\d\s.-]{7,20}$/;
+
+const submissionSchema = new mongoose.Schema(
+  {
+    source: {
+      type: String,
+      enum: SOURCES,
+      required: [true, "Source is required"],
+    },
+    pageUrl: {
+      type: String,
+      trim: true,
+      default: "",
+    },
+    name: {
+      type: String,
+      required: [true, "Name is required"],
+      trim: true,
+      maxlength: [160, "Name cannot exceed 160 characters"],
+    },
+    email: {
+      type: String,
+      default: "",
+      trim: true,
+      lowercase: true,
+      validate: {
+        validator(value) {
+          return !value || emailPattern.test(value);
+        },
+        message: "Please enter a valid email",
+      },
+    },
+    phone: {
+      type: String,
+      trim: true,
+      default: "",
+      validate: {
+        validator(value) {
+          return !value || phonePattern.test(value);
+        },
+        message: "Please enter a valid phone number",
+      },
+    },
+    payload: {
+      type: mongoose.Schema.Types.Mixed,
+      default: {},
+    },
+    status: {
+      type: String,
+      enum: STATUSES,
+      default: "new",
+    },
+    internalNote: {
+      type: String,
+      trim: true,
+      default: "",
+      maxlength: [2000, "Internal note cannot exceed 2000 characters"],
+    },
+    ipAddress: {
+      type: String,
+      default: "",
+    },
+    userAgent: {
+      type: String,
+      default: "",
+    },
+  },
+  {
+    collection: "submissions",
+    timestamps: true,
+    autoCreate: false,
+    autoIndex: false,
+  },
+);
+
+submissionSchema.statics.SOURCES = SOURCES;
+submissionSchema.statics.STATUSES = STATUSES;
+
+module.exports = mongoose.model("Submission", submissionSchema);