Documentation Update - Reverse Proxy configuration for Seer

[TN-1]

Description

When I first connected Fladder to Seer it was not able to auth. Piecing together a few different issues and some help from an LLM I was able to work out the correct CORS headers to apply in traefik. It would be helpful to edit install.md to include this information so its more obvious in the future.

Traefik dynamic config:

http:
  middlewares:
    seerr-cors:
      headers:
        accessControlAllowOriginList:
          - "https://fladder.example.xyz"
        accessControlAllowMethods:
          - "GET"
          - "POST"
          - "PUT"
          - "DELETE"
          - "OPTIONS"
        accessControlAllowHeaders:
          - "Content-Type"
          - "Authorization"
          - "X-Requested-With"
          - "X-Api-Key"
        accessControlAllowCredentials: true

This middleware is then applied to the Seerr router.

Fladder and Seerr then talk happily.

If others could post their configs for other reverse proxies that would be useful