chore: remove the dbt plugin and alpine-dbt docker image

[klesh]

Summary

Removes the deprecated dbt plugin from the backend, its alpine-dbt Docker image, the corresponding config-ui example entry, and the dbt-mysql / dbt-postgres Python dependencies. Updates the plugin registration test lists (table_info_test.go, migration_schema_test.go, server_startup_test.go) accordingly.

Closes #8970.

Background

Per the issue, the dbt plugin architecture relies on passing external user-supplied configurations to external CLI tools (git and dbt), exposing the server to SSRF and CLI argument manipulation. Rather than maintaining complex input-filtering rules for this changing command-line surface, the plugin is being deprecated and removed from the codebase.

Scope

• Backend plugin: Deleted backend/plugins/dbt/ (9 files: entry point, impl, api/swagger, tasks/convertor, tasks/git, tasks/options, tasks/options_test, tasks/task_data, README)
• Docker image: Deleted devops/docker/alpine-dbt/ (Dockerfile + README)
• Config-UI: Deleted example/dbt.ts and removed the dbt entry from example/index.ts
• Python deps: Removed dbt-mysql and dbt-postgres from backend/python/requirements.txt
• Tests: Removed dbt imports and registrations from table_info_test.go, migration_schema_test.go, and server_startup_test.go

Verification

• go vet passes for the edited packages (only a pre-existing, unrelated libgit2 system-dependency error remains)
• tsc --noEmit passes for config-ui
• rg for all dbt plugin symbols (plugins/dbt, dbt/impl, alpine-dbt, dbt-mysql, dbt-postgres, DbtOptions, DbtTaskData, DbtConverter, impl.Dbt, dbt.Dbt) returns no matches

Checklist

• The backend builds without referencing the dbt package
• No regressions introduced to other core backend plugins or connection management APIs
• Plugin registration test lists updated to stay in sync with backend/plugins/ directories

[Startrekzky]

lgtm