feat: Add deployment_key table, entity, repository, and migration

[BGZStephen]

Summary

Adds the deployment_key table as pure DB infrastructure for the encryption key rotation epic (T01). No business logic is included.

What's added:

• DeploymentKey TypeORM entity extending WithTimestampsAndStringId (id, createdAt, updatedAt inherited)
• Columns: type (varchar 64), value (text), algorithm (varchar 20, nullable), status (varchar 20), deprecatedAt (datetime, nullable)
• DeploymentKeyRepository with findActiveByType, findById, and findAllByType methods
• Migration 1777000000000-CreateDeploymentKeyTable registered for both SQLite and PostgreSQL
• Partial unique index IDX_deployment_key_data_encryption_active on type where status = 'active' AND type = 'data_encryption' — enforces at most one active data_encryption key at a time

Related Linear tickets, Github issues, and Community forum posts

https://linear.app/n8n/issue/IAM-484

Review / Merge checklist

• PR title and summary are descriptive. (conventions)
• Docs updated or follow-up ticket created.
• Tests included.
• PR Labeled with Backport to Beta, Backport to Stable, or Backport to v1 (if the PR is an urgent fix that needs to be backported)
• I have seen this code, I have run this code, and I take responsibility for this code.

[codecov]

Codecov Report

❌ Patch coverage is 58.06452% with 13 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...n/db/src/repositories/deployment-key.repository.ts	33.33%	8 Missing ⚠️
...s/common/1777000000000-CreateDeploymentKeyTable.ts	16.66%	5 Missing ⚠️

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