cloud: revise Premium DM wording

[Oreoxmt]

This PR restructures how Premium data migration content is organized so that Premium follows the same documentation pattern as Dedicated and Essential, and it folds Premium-specific details into the shared data migration docs instead of keeping a separate Premium-only overview file.

[Oreoxmt]

@alastori Please help confirm the quota for the Premium.

[alastori]

Code-verified (AI-assisted): Premium DM has no enforced quota. dataflow-service-ng/app/controllers/premium_dm/premium_dm.go validateCreateTaskReq and isWhitelisted are no-ops, no GetPremiumDMQuota RPC in proto, and no quota field in CreatePremiumMigrationReq. Confirming with Leon; if a quota is enforced, we will document it in a follow-up PR. For now, the Premium variant of this section is dropped on premium-data-migration-8.5.

[alastori]

Hi @Oreoxmt, thanks for the structural review. As discussed in DM, your direction (folding Premium DM into the canonical Cloud DM docs alongside Dedicated and Essential, and removing the standalone overview) is the right call long-term, and I have integrated it into PR pingcap#22821 via cherry-pick of 78f85f5d5, with follow-up commits to reconcile a few points where the structural copies did not match the current Premium implementation.

Below is a summary of what changed and why. I have closed this PR in favor of the integrated commits on premium-data-migration-8.5.

What was kept from your PR

• The standalone tidb-cloud/premium/premium-data-migration.md is deleted.
• Premium content lives in <CustomContent plan="premium"> blocks alongside Dedicated and Essential variants in the two canonical docs.
• The TOC moves the Cloud DM docs from the Premium-specific section to the shared "Migrate Data into TiDB Cloud" section.
• Public Preview note, supported sources table, physical/logical mode block, safe-mode 60s block.

What was adjusted in PR pingcap#22821 and why

1. Connectivity method (Premium): Public + Private Link, not Public-only. Your PR documented Premium as Public-only. Premium DM now supports AWS PrivateLink for source connectivity, sharing the Private Endpoint UI with Premium Changefeed. Backend: dataflow-service PR dashboard: update grafana-overview-dashboard.md pingcap/docs#3347 (feat(premium-dm): Support PrivateLink). UI: dbaas-ui PR adopters: add case study links (#4742) pingcap/docs#4755 (feat/premium-dm-private-endpoint). I added a Premium connection-methods table (Public for all providers; Private Link AWS-only), a Step 2 conditional follow-up that handles both modes (Hostname or IP address for Public; Private Endpoint dropdown with inline "Create a Private Endpoint here" link for Private Link), and a Premium variant of the post-validate step covering both the Public IP allowlist and the AWS endpoint connection acceptance. UI strings are verified end-to-end against the wizard on dev.

2. Premium has no Restart action. The plan-agnostic "click the Restart button" recovery instruction in "Limitations of incremental data migration" was rendering for Premium readers. Premium DM proto exposes only Pause, Resume, and Delete with no Restart RPC, so the instruction was not actionable. I wrapped the bullet to render only for Dedicated and Essential. (Aside: same gap may exist for those tiers per their protos. I scoped the fix to Premium for this PR; happy to investigate the broader question separately.)

3. Premium quota TODO dropped, not filled. The placeholder "You can create up to xxx TODO migration jobs on Premium instances" block was removed. Per the code, Premium DM has no enforced quota: dataflow-service-ng/app/controllers/premium_dm/premium_dm.go validateCreateTaskReq and isWhitelisted are no-ops, no GetPremiumDMQuota RPC in proto, and no quota field in CreatePremiumMigrationReq. I am confirming with Leon; if a quota is enforced, we will document it in a follow-up PR.

4. Alibaba Cloud RDS / PolarDB-X limitations duplicated for Premium. Your PR kept these inside the Essential variant only. They are source-side constraints (hidden primary key in binlog, schema keywords) that apply regardless of TiDB Cloud tier. Duplicated as a Premium variant so Premium readers see them. (Dedicated also supports Alibaba RDS per the supported-sources table but does not show these limitations either; that is a separate gap.)

5. Incremental doc Premium variants expanded. Your additions in migrate-incremental-data-from-mysql-using-data-migration.md documented Premium as Public-only (Connectivity method "Select Public", Hostname-only follow-up, Public-only validate step). I rewrote the three Premium variants to mirror the Public + Private Link pattern in the existing-data canonical doc.

Verification trail

• Wizard run on dev cluster keep-720h-cslb (us-east-1, 2026-04-30) to verify field labels and dropdown options for Step 1.
• Code references: dataflow-service-ng/app/controllers/premium_dm/, dataflow-service-ng/idl/dataflow/premium_dm/v1beta2/premium_dm.proto, dbaas-ui/src/dbaas/screens/DataMigration/steps/Step1/, dbaas-ui/src/dbaas/screens/NextGen/premium/networking/private-endpoint-changefeed/EntryCard.tsx.
• Architecture summary: dataflow-service/.domain-knowledge/ng-premium-dm.md section 12 (PrivateLink Integration via shared BizSinkEndpointUsage).

Thanks again for the restructure. Closing this PR.