Migrate from User to Account domain concept

Renames core domain entities and services from "User" to "Account"

Refactors project-wide namespaces, classes, and database tables to use "Account" terminology
Updates related components, services, and database schema to reflect new domain naming
Standardizes naming conventions across authentication and organization setup features

Database/Core/AccountService.cs

@@ -0,0 +1,79 @@
+using Insight.Database;
+using PlanTempus.Core;
+using PlanTempus.Core.Entities.Accounts;
+using System.Data;
+
+namespace PlanTempus.Database.Core
+{
+    public class AccountService
+    {
+        public record AccountCreateCommand(string CorrelationId, string Email, string Password);
+
+        private readonly IDbConnection _db;
+
+        public AccountService(IDbConnection db)
+        {
+            _db = db;
+        }
+
+        public async Task CreateAccount(AccountCreateCommand command)
+        {
+            var account = new Account
+            {
+                Email = command.Email,
+                PasswordHash = new SecureTokenizer().TokenizeText(command.Password),
+                SecurityStamp = Guid.NewGuid().ToString(),
+                EmailConfirmed = false,
+                CreatedDate = DateTime.UtcNow
+            };
+
+            var accountId = await _db.ExecuteScalarAsync<int>(@$"
+                INSERT INTO accounts (email, password_hash, security_stamp, email_confirmed, created_at)
+                VALUES (@Email, @PasswordHash, @SecurityStamp, @EmailConfirmed, @CreatedDate)
+                RETURNING id", account);
+        }
+
+        public async Task CreateOrganization(int accountId, string organizationConnectionString)
+        {
+            var schema = "dev";
+
+
+            using var transaction = _db.OpenWithTransaction();
+            try
+            {
+                // Create organization
+                var organization = new Organization
+                {
+                    ConnectionString = organizationConnectionString,
+                    CreatedDate = DateTime.UtcNow,
+                    CreatedBy = accountId,
+                    IsActive = true
+                };
+
+                var organizationId = await _db.ExecuteScalarAsync<int>(@$"
+                INSERT INTO {schema}.organizations (connection_string, created_date, is_active)
+                VALUES (@ConnectionString, @CreatedDate, @IsActive)
+                RETURNING id", organization);
+
+                // Link account to organization
+                var accountOrganization = new AccountOrganization
+                {
+                    AccountId = accountId,
+                    OrganizationId = organizationId,
+                    CreatedDate = DateTime.UtcNow
+                };
+
+                await _db.ExecuteAsync(@$"
+                INSERT INTO {schema}.account_organizations (account_id, organization_id, created_date)
+                VALUES (@AccountId, @OrganizationId, @CreatedDate)", accountOrganization);
+
+                transaction.Commit();
+            }
+            catch
+            {
+                transaction.Rollback();
+                throw;
+            }
+        }
+    }
+}