Make deployment region configurable via gapp setup

[krisrowe]

Region is hardcoded as us-central1 in deploy.py (line 96). It should be configurable via gapp setup --region and stored in solutions.yaml.

Current State

• One hardcoded location: deploy.py:96 sets region = "us-central1" — affects Artifact Registry paths and is passed implicitly to Terraform via image URL
• Terraform module already accepts a region variable with default "us-central1", but it's never populated from Python
• solutions.yaml stores only project_id and repo_path — no region field
• context.resolve_solution() returns {name, project_id, repo_path} — no region propagation
• GCS buckets are multi-regional (--location us), region-agnostic naming — safe
• Artifact Registry image paths embed the region (us-central1-docker.pkg.dev/...) — hard dependency

What Needs Updating

Component	Change Needed
solutions.yaml	Add optional region field (default us-central1)
context.resolve_solution()	Propagate region through context dict
deploy.py	Read region from context instead of hardcoding
_build_tfvars()	Pass region to Terraform
gapp setup	Accept --region flag, store in solutions.yaml
gapp solutions list	Display region
gapp status	Show region in output

What's Safe / Unaffected

• GCS buckets — multi-regional, no region in naming
• Terraform state — region-agnostic backend config
• Secret Manager — project-scoped, not region-scoped
• GCP discovery (label-based) — no region filters

Key Risks

• Artifact Registry image paths embed region. Changing region on an existing deployment orphans old images. Must either prevent mid-lifecycle region changes or implement migration logic.
• gapp solutions restore (not yet implemented) and any future import/reattach flows will need to discover or infer region from existing infrastructure
• gapp status currently reads TF outputs from remote state — region-agnostic. But if we ever add gcloud run queries directly, they'll need region.
• Cloud Build currently doesn't specify region (uses default). May want to align with deploy region.

Recommendation

• Make --region required on gapp setup when no region is already stored
• Default existing solutions to us-central1 for backward compatibility
• Add validation to reject region changes after first deploy (Artifact Registry images are region-locked)
• Keep GCS buckets multi-regional for now — no reason to couple bucket location to service region