Adds Authenticator.RequireBearer — http middleware that gates downstream
handlers on a valid broker access token.
Lookup path:
1. Read Authorization: Bearer <token> header.
2. SHA-256 the token, query access_tokens by token_hash.
3. Reject expired or revoked rows.
4. Build a Session (client_id, forgejo user info, upstream token,
scopes) and attach to r.Context() under a typed key.
Downstream handlers (the MCP endpoint shipping in 5a) read the
upstream Forgejo token via SessionFromContext to spawn forgejo-mcp
subprocesses scoped to the right user.
Failures emit 401 with an RFC 6750 §3 WWW-Authenticate header carrying
distinct error codes (invalid_request for missing/malformed headers,
invalid_token with reason=expired/revoked/unknown for token problems).
The body stays empty so a confused browser doesn't render auth errors;
all detail rides in the header where compliant clients look for it.
Tests: 90.9% on RequireBearer, 91.7% on lookupSession. Covers valid
token, missing/wrong-scheme/empty Authorization, unknown token,
expired token (clock-advanced past AccessTokenTTL), revoked token (via
the public /oauth/revoke endpoint).
Closes forgejo-mcp-broker-ytw.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
