Make request header read buffer size configurable (fixes 431 Request Header Fields Too Large)

[neverlless]

Problem

Fiber's default ReadBufferSize is 4096 bytes, so mokey rejects any request whose headers exceed 4KB with 431 Request Header Fields Too Large:

level=error msg="Request Header Fields Too Large" code=431 path=/ username="<nil>"

This happens easily when mokey runs on a (sub)domain shared with other applications that set large cookies, or behind auth proxies that add sizable headers. Reported in #122.

Fix

• Add a server.read_buffer_size config option wired to fiber.Config.ReadBufferSize, defaulting to 16384.
• Document the option in mokey.toml.sample.

Testing

Deployed in production behind Traefik where users' shared-domain cookies previously triggered 431s. Verified in the running container:

# before (v1.0.2 base): 5KB cookie
curl -H "Cookie: junk=<5KB>" http://localhost:8866/   -> 431
# after: 5KB and 12KB cookies
curl -H "Cookie: junk=<5KB>" http://localhost:8866/   -> 302
curl -H "Cookie: junk=<12KB>" http://localhost:8866/  -> 302

Fixes #122