feat(terraform): AWS Fargate deployment module

Adds a terraform/ directory with an opinionated module that deploys
shared-memory to ECS Fargate behind an ALB. The module assumes the
operator already provides the VPC, RDS Postgres, ACM cert, ECR images,
and OIDC clients, and creates everything else: ECS cluster + services,
ALB, Service Connect namespace for app-embedder discovery, EFS-backed
model cache for the embedder, Secrets Manager entries, IAM roles,
CloudWatch log groups, and a one-shot migrator task definition.

Includes examples/basic/ with a worked invocation and a README covering
prerequisites, quick start, the post-apply migrator run, image updates,
DNS setup, and a security note. Main README gains a short Mode C
pointer to the terraform/ guide.

Validated with `terraform fmt -check -recursive` and
`terraform validate` against AWS provider 5.x.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

terraform/versions.tf

@@ -0,0 +1,10 @@
+terraform {
+  required_version = "~> 1.5"
+
+  required_providers {
+    aws = {
+      source  = "hashicorp/aws"
+      version = "~> 5.0"
+    }
+  }
+}