various improvements

[schtobia]

Hi all, I've made some improvements: https://github.com/schtobia/timew-sync-server/tree/dev/for_upstream
To summarize my work:

• Added a golangci-lint config and fixed all the resulting warnings
• Fixed the loop condition in GetFreeUserID (was i > 0, now i >= 0) and added tests for user management.
• SolveConflict now computes a net diff and applies changes via a single ModifyIntervals call instead of separate add/remove loops.
• Storage error handling: returns wrapped errors instead of log.Fatalf during DB initialization/migrations and checks rows.Err() after iteration.
• Dependency injection: replaced storage.GlobalStorage and sync.PublicKeyLocation with a sync.ServerConfig struct that carries the storage backend, key directory, and auth settings.
• HTTP hardening: restricted /api/sync to POST, and configures the server with sensible read/write/idle timeouts.

Is this of any interest, should I issue an PR? If so, should I put this in one PR or should I open several?

[edit] Also, I have some more changes prepared if they are of any interest - one of them fixes #67.

[joachimschmidt557]

Hi, wow, that looks great! We would welcome any of these changes. Feel free to open separate pull requests for independent fixes/improvements :)

[schtobia]

The first batch of PRs are out, unfortunately commit c3de2e8 cannot be set up easily independent from the others, I would put this as a PR once the others are successfully merged.

[joachimschmidt557]

Thanks! As a heads-up, in the back of our heads, we are thinking about moving the handling of users and their keys away from the filesystem and into the database. 0e3913e was a first step into this direction.