fix: distinguish project-invitation email subject from plain org invitation

[jhosepmyr]

Description

Follow-up to #73 (merged before this commit was pushed to the branch). Real end-to-end verification against a running app + Mailpit found that sendProjectInvitationEmail's subject, heading, and preheader were copy-pasted verbatim from sendInvitationEmail — a recipient scanning their inbox list couldn't tell a project invite apart from a plain org invite; the only difference was buried in an extra body paragraph.

Both the subject line and the in-email heading/preheader now name the project explicitly.

Bounded context / area: iam

Related issue / US: Follow-up to #73

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Type of Change

• feat — new feature
• fix — bug fix
• refactor — code change without behavior change
• test — tests only
• docs — documentation only
• build / ci — build, dependencies, or CI/CD
• chore — maintenance

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Checklist

• The PR targets develop (not main)
• Branch name follows feature/*, bugfix/*, or hotfix/*
• Commits follow Conventional Commits
• ./gradlew build passes locally (compile + tests)
• New cross-module access respects module boundaries — change stays inside iam.infrastructure.email
• Added/updated tests for the change — existing SmtpEmailAdapterTest content assertions still pass; no new test added since the fix is subject-line/heading text, not new logic to unit test in isolation (covered by the E2E Mailpit verification instead)
• No secrets, credentials, or .pem keys are committed
• CHANGELOG.md updated under [Unreleased] — not done, this is a same-day fix to an unreleased entry already documented in feat: redesign transactional email templates with 2026 best practices #73; no separate entry needed
• Database changes are expressed as Flyway migrations — N/A

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How to Test

1. Trigger a project invitation (POST /api/organizations/{orgId}/projects/{projectId}/members/invite) and a plain org invitation (POST /api/organizations/{orgId}/members) side by side
2. In Mailpit (or any inbox), confirm the two now show visibly different subjects — the project invite names the project, the org invite doesn't

Notes / Screenshots (optional)

Found via real E2E verification (app + Mailpit, all 5 email flows triggered through actual API calls) after #73 merged — not caught by unit tests since they only assert body content, not subject lines.