Purge by cache tags can quickly pile up a backlog in some scenarios

Problem/Motivation

I can see that the purge by cache tags is piling up multiple queues of items e.g. if a client resaves the same item, or two separate items e.g. both result in node_list tag needing a clear. The queue can quickly get out of hand with duplicates particularly in a hosting like Upsun where the frequency of cron jobs is maximum once every 5 minutes.

Steps to reproduce

1. Set up purge by cache tag using queue mechanism.
2. Change a few items or same item multiple times.
3. Run cron very seldom
4. Review queue

Proposed resolution

Cache tags invalidated while auto-purge is queueing are merged into a deduplicated pending set instead of creating a queue item per invalidation, so repeated invalidations of the same tag no longer pile up duplicate queue items. Cron drains the pending set FIFO into queue items batched within Cloudflare's per-request tag limit. A new "Queued Tags" page shows what's pending/queued and lets an admin trigger a drain manually. Advisory warnings flag when the queue may be growing faster than it can be purged. Full details in the module's CHANGELOG.

Update the queue purge system to review existing queued items and combine into larger requests within Cloudflare's limits in a FIFO (first in first out) way. Cloudflare's rules on this: https://developers.cloudflare.com/cache/how-to/purge-cache/purge-by-tags/ ie

• A single HTTP response can have more than one Cache-Tag HTTP header field.
• The minimum length of a cache-tag is one byte.
• Individual tags do not have a maximum length, but the aggregate Cache-Tag HTTP header cannot exceed 16 KB after the header field name, which is approximately 1,000 unique tags. Length includes whitespace and commas but does not include the header field name.
• For cache purges, the maximum length of a cache-tag in an API call is 1,024 characters.
• The Cache-Tag HTTP header must only contain printable ASCII encoded characters.
• Spaces are not allowed in cache-tags.
• Case is not sensitive. For example, Tag1 and tag1 are considered the same.

Remaining tasks

Merge request + test coverage

User interface changes

N/A

API changes

N/A

Data model changes

N/A