docs: Add a Serverpod Cloud database backups page

[developerjamiu]

Summary

Adds a Serverpod Cloud Database backups concept page documenting the new scloud db backup and scloud db schedule commands: manual snapshots, scheduled backups, restore, plan gating, and pricing.

• New page: cloud_docs/concepts/database-backups.md (Overview, when-to-use, the five operations, Pricing, Limits, Related).
• Reconciles the outdated 2-line ## Backups stub on the Database page (it claimed backups were automatic and needed no configuration) into a short pointer to the new page.
• Renumbers two sidebar positions so backups sits directly after Database (personal-access-tokens 6→7, deployment-hooks 7→8).

Do not merge until the feature is released

The backup CLI is on main in the cloud repo but not yet in a released scloud, and the web console is still in progress (CLD-505). Keep this open for review and merge once the feature ships (targeting ~week of Jul 20). Console sections will be added in a follow-up once the console lands.

Verification

Every claim was checked against the cloud repo at origin/main (backup CLI shipped in #2090):

• CLI surface, flags, defaults, and output/confirmation wording read from the command source, not written from memory.
• Growth-plan gating and the split of gated vs. any-plan commands confirmed in the databases domain/endpoint code.
• Pricing ($6 / 10 GB, separate metered charge) and the 100 manual-snapshot cap confirmed in the billing and policy config.
• npm run build passes (no broken links or anchors).

Notes for reviewers

• Downgrade with backups is documented as if enforced (a Growth→smaller-plan change is blocked while backups exist). This behavior is intended but not yet implemented; documenting it as in effect was agreed with the platform team. Worth a final confirm before merge.
• Scheduled-snapshot retention default is described as "the platform's default retention" rather than a specific number, matching the CLI's own wording. If we want to state the exact value, that's the one detail still to pin.
• Pricing rate is stated on the page and linked to the plans page. Once a dedicated plans/pricing page exists, the exact rate can move there and this page can link out.
• The provider is deliberately never named; backups are described as a feature of the managed database.

[Zfinix]

LGTM 🚀 great work on this.

nit (non-blocking): the $6 / 10 GB rate will drift as plans change — consider dropping the exact number here and letting the plans page own it, keeping the "separate metered charge, same rate as database storage" framing.

[Isakdl]

Good work! I like it,

I do have some thoughts and suggestions. I will probably not be available when you will merge this, so you can dismiss my needed rereview.

[christerswahn]

Added responses to a couple of comments.

[christerswahn]

LGTM

Isak asked Christer to review as he is currently away