crates/models: resolve capture interval from the connector tag

[nicolaslazo]

Description:

connector_tags.default_capture_interval has existed for a long time but nothing ever applied it.

interval becomes Option<Duration>, with unset meaning "resolve at build time". If the model does not provide its own interval, then the connector tag's takes precedence over the global fallback.

Build-time resolution costs propagation latency: a changed tag default reaches an existing capture only when that capture is next built. That happens automatically within twenty days via the periodic touch publication, and sooner if anything else republishes the task.

Workflow steps:

(How does one use this feature, and how has it changed)

Documentation links affected:

(list any documentation links that you created, or existing ones that you've identified as needing updates, along with a brief description)

Notes for reviewers:

Tested locally using unit tests.

[github-actions]

🚀 Preview deployed to https://docs.estuary.dev/pr-preview/pr-3333/

📄 Changed pages:

• /concepts/captures/
• /reference/Connectors/capture-connectors/

[jgraettinger]

What about resolving this from the Spec response, Open response, or an image label? For example we change validation semantics so that "unset" propagates down into the built spec, and then have the reactor choose the restart interval through a negotiation with the started connector.

It seems circuitous to have connector_tags control this, given it's a property of the connector, and I have uncertainty about how fit-for-purpose connector_tags will remain going forward.

[jwhartley]

Docs have moved, and this PR straddles the split. The code belongs here; the
doc files no longer do.

estuary/flow's site/ tree is no longer published. docs.estuary.dev is
served from estuary/docs (platform docs, under content/), which aggregates
connector pages from estuary/connectors (under docs/reference/Connectors/)
at build time.

Suggested split:

• Keep the code changes in this PR and drop the doc files from it.

• Open the doc half against estuary/docs, where these paths map across:

  • site/docs/concepts/captures.md becomes content/concepts/captures.md

Flagging it because the doc half would otherwise merge into a tree nobody
serves. The "docs.estuary.dev: Deploy" check will still go green, which is
deliberate (it keeps flow's gh-pages rollback artifact current until
2026-09-30) and is not a signal that the doc change shipped.

Context: migration announcement.