[Story]: Connect sources enabled-by-default (simplify connect → ingest workflow)

[adoLime]

User Story

As a PM/Admin, I want newly connected GitHub repositories to be ready for ingestion without an extra enable step, so that connecting a source is a single action instead of a multi-step workflow.

Context & Motivation

Today, connecting a GitHub repo creates a GithubRepositoryConnection with sourceEnabled = false. The PM/Admin must then separately toggle the source on before any artifacts can be ingested. The same applies to Jira instances (JiraInstance.sourceEnabled = false). For the user this is a long, click-heavy workflow: connect → open sources → find the new one → enable → wait. We want to remove the unnecessary intermediate step so the act of connecting a repo implies "ingest from it".

In Scope

• Make newly connected sources enabled-by-default, so artifacts can be ingested directly after connecting (GitHub, and Jira for consistency).
• Decide and implement the desired behavior: e.g. auto-enable on connect, or a single "Connect + enable" action, or a bulk "enable all" affordance — whichever fits the existing connector flow best.
• Keep the ability to disable a source afterwards (no loss of existing control).

Acceptance Criteria

• After connecting a GitHub repository, the source is enabled by default and artifacts are ingested without a separate enable step.
• After connecting a Jira instance (if in scope), the source is enabled by default as well.
• A user can still disable a connected source later, and a disabled source stops feeding ingestion as before.
• Existing connections are not silently re-enabled by this change (no unintended backfill).
• The connect flow in the UI reflects the new behavior (no dead "enable" step that the user must click just to make ingestion start).
• Tests cover the new default-on behavior and the still-working manual disable path.
• Documentation / changelog notes that newly connected sources are enabled by default.