Sped up email renderer

[EvanHahn]

no ref

This should be a performance-only change. This patch makes the following changes:

1. Only read .hbs files once, and only compile the Handlebars template once.
2. Read .hbs files in parallel, for performance.
3. Small: don't re-convert a string unnecessarily. (We were reading it as a UTF-8 string, then converting it back to a buffer, then re-reading that buffer as a UTF-8 string.)
4. Small: don't keep useless references around.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 1658ad54-d62a-412b-84de-5318bbe61f74

📥 Commits

Reviewing files that changed from the base of the PR and between a3b8016 and fc6f56a.

📒 Files selected for processing (1)

• ghost/core/core/server/services/email-service/email-renderer.js

🚧 Files skipped from review as they are similar to previous changes (1)

• ghost/core/core/server/services/email-service/email-renderer.js

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Walkthrough

The EmailRenderer was refactored to lazy-compile Handlebars templates. The private fields #handlebars and #renderTemplate were removed and replaced with #compiledHandlebarsRendererPromise. Two new private methods, #compileHandlebarsRenderer() and #getCompiledHandlebarsRenderer(), handle asynchronous compilation and memoization. The public method renderTemplate(data) now awaits and invokes the compiled renderer instead of using a precompiled template. A type annotation for TemplateDelegate was added.

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Sped up email renderer' directly captures the main objective of the PR—a performance improvement to the email renderer through lazy compilation and caching.
Description check	✅ Passed	The description clearly explains the performance-focused changes including lazy compilation, parallel file reading, and elimination of unnecessary conversions, all aligned with the code modifications.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

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🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch speed-up-email-renderer

📝 Coding Plan

• Generate coding plan for human review comments

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