[api2] events: add source_conversation_seen event

[vickiniu]

Adds a source_conversation_seen event that marks all items in a source conversation as seen before a given interaction_count. This allows the client to update potentially large conversations with a single event.

Test plan

Checklist

This change accounts for:

• any required additional documentation
• any necessary AppArmor changes (added or removed application files)
• any impact on new SecureDrop installs and upgrades
• our dependency update policy

[Copilot]

Pull request overview

Adds an API2 event intended to let clients mark a whole source conversation as “seen” up to a given interaction_count, reducing the need to send many per-item events for large conversations.

Changes:

• Adds a new API2 event data payload for “conversation seen” with upper_bound validation.
• Registers and implements a server-side handler to mark qualifying items as seen.
• Adds an integration test covering OK / AlreadyReported / Gone behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 6 comments.

File	Description
securedrop/journalist_app/api2/types.py	Introduces a new event type + payload, and wires it into EVENT_TYPES.
securedrop/journalist_app/api2/events.py	Registers the new event handler and implements “mark conversation seen” processing.
securedrop/tests/test_journalist_api2.py	Adds test coverage for processing the new event through /api/v2/data.

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[legoktm]

test_api2_source_conversation_seen is failing with:

>           response = app.post(
                url_for("api2.data"),
                json={"events": [asdict(event)]},
                headers=get_api_headers(journalist_api_token),
            )

...
                for uuid, item in result.items.items():
>                   response.items[uuid] = item.to_api_v2(minor) if item is not None else None
E                   AttributeError: 'str' object has no attribute 'to_api_v2'

[legoktm]

This is great, thanks :)