Team chat: internal-only comments and a team feed

Adds a parallel comment lane on every post that is visible only to org/project members - separate from the public discussion thread end-users see. Each post gets a "Team" tab next to "Public" comments. Plus a project-wide /team feed that aggregates the latest internal threads across all posts so admins can scan recent internal activity without clicking into each card.

Schema: extend comments with visibility enum (public, internal). RLS: internal comments only readable/writable by project members. UI: tabbed thread on post detail, plus a new project-level Team page. Out of scope: real-time presence (separate card), DMs, slash-commands, file uploads beyond what posts already allow.