feat(vision): add rollout ADR and default-off flags

[a-klos]

This pull request introduces infrastructure changes to support a safe, flag-gated rollout of vision embedding and image-native retrieval capabilities in the RAG platform. The changes add new configuration options and config maps for vision features, all defaulting to disabled, enabling a staged and reversible deployment process.

Key changes:

Vision Feature Flag Rollout and Documentation

• Added an ADR (ADR-vision-embedding-rollout.md) outlining the staged rollout plan, feature flags, and telemetry requirements for vision embedding support.

Helm/ConfigMap Infrastructure for Vision Flags

• Added new config map templates for vision-specific environment variables in backend, admin-backend, and extractor Helm templates (_backend_helpers.tpl, _admin_backend_and_extractor_helpers.tpl). [1] [2]
• Updated deployment YAMLs to mount the new vision config maps in backend, admin-backend, and extractor deployments. [1] [2] [3]
• Created new config map YAMLs for vision environment variables in backend, admin-backend, and extractor. [1] [2] [3]

Default Values and Environment Variable Wiring

• Added vision environment variable sections with all vision flags set to false by default in values.yaml for backend, admin-backend, and extractor. [1] [2] [3]