[FEATURE] Support associating policies with tags

### Describe the feature

Support associating policies with tags so a policy can be linked to a tag and then applied to metadata objects through the tag relationship.

### Motivation

PR #12177 defines the Policy -> Tag -> Metadata Object model. The server needs persistent storage and REST APIs for the direct Policy <-> Tag association before higher-level inheritance/resolution can build on it.

### Describe the solution

Add a policy_tag_relation_meta table, relational mapper/service support, dispatcher/manager APIs, REST endpoints to list tags for a policy and list/associate policies for a tag, and targeted tests.

### Additional context

Based on the design in #12177.

Start with the design in issue #12177, then inspect the existing relational mapper/service, dispatcher and manager APIs, and REST endpoint patterns. Implement the policy_tag_relation_meta persistence and Policy–Tag association operations described here, with targeted tests covering listing and association behavior; done means both REST listing paths work and the relation is stored.

Written by the indexing model from the issue text.