docs: research Terraform HTTP backend compatibility

[nathanhruby]

Summary

• document the Terraform and OpenTofu HTTP backend wire contract United must preserve
• cover state retrieval/update/deletion, lock/unlock semantics, accepted statuses, lock IDs, authentication, headers, retries, and compatibility implications
• record implementation constraints and newly surfaced design questions for the PocketBase replatform

Supports Document Terraform HTTP backend compatibility requirements in Plan United’s PocketBase-only replatform.

Branch note

This branch is stacked on research/pocketbase-consistency, so the PR currently includes that branch’s research commit. Merge the PocketBase consistency PR first; this PR will then reduce to the Terraform HTTP contract commit aa7f3385.

Validation

• go test ./...
• mise run check:ci