Move SecretEntityType off EntityType onto DanglingFieldType

SecretEntityType is declared as an EntityType and wired as a system entity group, but there is no secrets table. Both of its actions — GetSecretStatusAction and ReencryptSecretsAction — reach the stored secrets of every encrypted column across the tables holding them, so no row of its own is ever named.

A secret is a field several entities carry, and the owner is therefore not one fixed entity kind. That is a DanglingFieldType.

Scope:
- Redeclare SecretEntityType as a DanglingFieldType in common/data/entity/secret.py and rename it accordingly
- Rewire the secret service off the system entity group onto a field group in manager/services/factory.py
- Update src/ai/backend/common/data/entity/KNOWLEDGE.md, which records this move as still pending

Done when:
- The type is a DanglingFieldType and answers None for its owner
- Both secret actions run through field wiring
- No unclassified entity type remains for secret

JIRA Issue: BA-7787

Read common/data/entity/secret.py to locate SecretEntityType and its actions, then inspect manager/services/factory.py for the current system-entity wiring. Update the entity declaration, field wiring, and src/ai/backend/common/data/entity/KNOWLEDGE.md; done means the type is a DanglingFieldType with no owner, both actions use field wiring, and no secret entity type remains unclassified.

Written by the indexing model from the issue text.