Require signed auth for feed writes

[lawyered0]

Summary:

• Require signed auth headers for write operations in /api/feed
• Add signed-request helpers in the Factory web API client
• Update Farcaster feed mutations to sign requests before posting

Security:
Previously, clients could spoof x-wallet-address and perform write actions on behalf of another user. This change requires a wallet signature for feed write endpoints.

Testing:

• Not run (auth plumbing changes only)

[lawyered0]

Why this should merge:

• Security: prior to this change, any client could spoof and perform write actions on .
• This change preserves all read paths and only hardens write endpoints.
• Client signing uses the existing message format already validated by the backend.

Repro (before fix):

• Send with any header to like on behalf of that address.

Now: write actions require a signed header set.