fix(api): reject a placeholder or too-short encryption key in production

[cavidelizade]

Summary

The production guard only checked that INSTANCE_ENCRYPTION_KEY was set, and .env.example ships it as change-me-generate-a-random-key. Copying the example to a real host and setting ENV=production passed the guard while encrypting all instance secrets (SMTP password, OAuth client secrets, GitHub App private key) under a publicly known key.

Linked issues

Closes #330

Type of change

• Bug fix (fix:)

Surface

• API (apps/api/)

What changed

Added isWeakSecret, and the production startup guard now rejects a set-but-weak INSTANCE_ENCRYPTION_KEY: the shipped placeholder values and anything shorter than 16 characters fail startup outside development, alongside the existing empty check.

Test plan

• go build, go vet, go test ./... green, including the new TestIsWeakSecret

Notes

The issue also mentions ENV defaulting to development and the key being read via os.Getenv instead of the Config struct. I kept this PR to the concrete "placeholder passes the guard" hole. Happy to follow up on the default and the config threading if you want them in scope.

AI assistance

• AI tools were used, tool(s): Claude Code (Opus 4.8), commits carry a Co-Authored-By: trailer

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📥 Commits

Reviewing files that changed from the base of the PR and between 61bc903 and 66fdb42.

📒 Files selected for processing (2)

• apps/api/internal/config/config.go
• apps/api/internal/config/config_test.go

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