Add crate feature flags to reduce compile footprint

[Thunder-Blaze]

Why this matters

• The crate exposes many modules by default; consumers with narrow needs compile more than necessary.

Proposed work

• Add feature flags per endpoint group (for example, media, user, forum, review).
• Keep a full default for compatibility, and allow opt-in minimal builds.

Acceptance criteria

• Crate compiles with selective endpoint features enabled.
• Documentation includes feature matrix.