fix(directory): synthesize internal directory member for email principals in web-only auth mode

[elf-mouse]

This commit enables project member search and invitation for email users when Slack directory sync is not enabled:

1. src/api/routes/directory.ts:

   • Fall back to synthesizing an internal directory member when q is an email address and no matching record exists in directory_members.

2. src/directory/postgres-directory-store.ts & src/directory/directory-store.ts:

   • Synthesize a valid internal DirectoryMember in get(principalId) for email addresses not present in the Slack directory table, preventing false invalid_member rejections during project member invitation.

3. test/directory-resolve.test.ts:

   • Add unit test verifying email address resolution fallback.

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