Support alternatives to Meilisearch

Problem

From skimming over the packages used in Fluxer as well as the current Docker Compile file provided, I've noticed that Fluxer seems only supports Meilisearch as the search backend for it. This could possibly be a bit too heavy for some users who'd wish to selfhost Fluxer or are running it alongside other services in constrained environments, one way or another, and would like to have search capabilities and plan to have sizable communities on their server.

Proposed solution

Compatibility with other search backends, or otherwise replacing Meilisearch, should probably be considered or implemented for the sake of self-hosting (or performance improvements/lightweightness in general) friendly. A few I could probably suggest that would probably work well in the usecase I described and from experience/the top of my head:

• Sonic
• PGroonga or/and TSVector- (For these options to work, Fluxer would *first* need to gain support for using PostgreSQL as the database over SQLite, currently asked in [#960](https://feedback.fluxer.com/p/960))