[Feature Request] Add security-relevant logging for authentication and document share links

Description Problem Paperless-ngx currently gives administrators no way to audit who accessed what: • Successful logins are not logged at all. Only failed login attempts are, and even those don't record the client's User-Agent. • Creating or deleting a document share link is not logged anywhere, even though a share link grants unauthenticated access to a document. • Accessing a share link (/share/<slug>) is completely unauthenticated by design, but there is no record of who used it, when, or whether the attempt was for an expired/unknown link, so there's no way to detect misuse or link enumeration. For self-hosted instances that may be exposed to the internet, this makes it hard to investigate suspicious activity (e.g. brute-force login attempts, a leaked share link being scraped, or someone guessing share link slugs). Proposed solution Add INFO-level logging, including client IP address (respecting X-Forwarded-For only from tru…