Harden session/ingestion token separation and gate admin routes

[cteyton]

Session and ingestion JWTs were signed with the same secret and verified
by the same code path with no claim distinguishing them. An ingestion
token pasted into the session cookie passed signature verification and
the middleware set user with undefined fields, granting access to every
session-protected endpoint (including token minting and integration
secrets). Mirror-side: a session JWT presented as a Bearer token would
have been rejected only because session JWTs happen to lack a jti.

Adds a type claim ("session" or "ingestion") that each middleware now
enforces explicitly, validates that session JWTs carry non-empty
sub/email/role, and introduces a requireAdmin middleware applied to
credential-bearing and write routes (tokens, integrations,
marketplace-sources, audit, plus the write operations on skills,
marketplaces, and plugins). Read-only analytics endpoints remain open to
any authenticated session.