[DT-4263][1/7] Record a send id on each email row - #3149

Addresses

https://broadworkbench.atlassian.net/browse/DT-4263

Security risk: low — adds a nullable send_id column to email_entity, filled with a random per-request UUID; no change to who receives email or who can read it.

Summary

email_entity has one row per recipient and nothing linking the rows of one send, so the email log (#3142) has to infer sends from a 10-minute gap between timestamps. Code review kept finding cases that inference gets wrong: overlapping batches, chains of repeat emails, sends split by the date range. This records the grouping when the emails are written instead.

• A request filter gives each request a fresh send id, held in a ThreadLocal and cleared on the response. EmailService.sendMessage writes it on every row it inserts.
• Every send loop runs inside one request, so one send is exactly the rows sharing a send_id, email type and entity reference, and no send loop has to change.
• Emails written outside a request, and every existing row, keep send_id null. The email log will keep the current timestamp rule for those rows.
• The one place email leaves the request thread is EmailNotifierResource.sendDailyMessages, which submits the expiration notices and both digests to an executor. Each of those jobs runs under its own send id through EmailSendId.run. The other executors (dashboards, dataset cleanup, reindexing) send no email.
• Also pins testFetchByCreateDate_with_limit_and_offset to UTC: it read the local date but built UTC midnights, so it failed between 8pm and midnight Eastern.

#3142 will switch its grouping to send_id where it's set once this merges.

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• Label PR with a Jira ticket number and include a link to the ticket
• Label PR with a security risk modifier [no, low, medium, high]
• PR describes scope of changes
• Get a minimum of one thumbs worth of review, preferably two if enough team members are available
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• Verify all tests go green
• Test this change deployed correctly and works on dev environment after deployment

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