feat: add CamSoda username scan module

[brunolm]

Adds a username scan module for CamSoda.

Namespaces

• /<user> — broadcasters (200, embedded user object with rich profile).
• Members have no public page; existence is confirmed via the signup availability check: GET /api/v1/user/check?type=username&value=<h> → {"match":1} taken / {"match":0} free (read-only, anonymous, covers both account types).

Correctness

• The check is case-insensitive and whitespace-trimmed, matching the site.
• match:1 is overloaded for sub-3-char reserved names (reports taken with no account behind it), so handles under 3 characters return error, not a verdict.
• The broadcaster user object is matched on username rather than taken first (recommendation carousels embed siblings).

Metadata: broadcasters get name, id, gender, created date, followers, rating, tags, avatar; members return type: member.

Transport: curl_cffi. Tested live against broadcasters and member accounts plus nonexistent handles. ruff + mypy clean.

[kaifcodec]

LGTM.