Syndication: enforce DZone Friday-only rule and prune stale queue tasks

[shai-almog]

Problem

DZone (like Foojay) is supposed to receive only the weekly Friday digest post. But queue_browser_syndication.py queued every eligible post, so Monday's modern-advertising-api was queued for DZone and the browser drain tool submitted it to moderation.

The earlier Friday-only edit also never actually took effect, for two reasons:

1. It was uncommitted/unpushed, so CI ran the old unfiltered code.
2. Even live, it only filtered at enqueue time — the offending DZone task was already in the queue, and the drain tool submits whatever is in the queue.

Fix (two layers)

• Enqueue filter: WEEKLY_FRIDAY_PLATFORMS gates which posts a Friday-only platform accepts (post.date.weekday() == 4), mirroring FoojayAdapter.accepts.
• Queue prune: every run now drops already-queued tasks that the rules no longer allow (queued before the filter existed, or added by hand). This is what actually prevents a stray task from reaching moderation. A task whose post date is unknown is kept — never silently dropped.

Testing

• Unit: stale Monday dzone: task → pruned; medium: → kept; Friday dzone: → kept; unknown slug → kept.
• End-to-end against a seeded queue: the Monday DZone task is pruned, Medium retained.
• py_compile clean.

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