Core: fixes and improvements

[brainless]

These are related to database, engine loading, etc.:

• Refactor new project/engine and opening existing project/engine to be simpler - Manually deleted project databases should not get recreated #195
• Reject API requests for project with no DB or record
• Stop engine when its project DB or record is deleted
• DB Lock issues - Lock issues #202
• Simplify storage on RocksDB
• If needed, persist to disk as a background job, decoupled from the in-memory engine
• If needed, change nodes and edges storage to vectors instead of hash map

Aside from these, the other (maybe more critical) task is to check why the graph relations seem to be broken sometimes - which is probably why I keep feeling that there is not enough relevant stuff on the UI. We need to decide this after running a few tests since these are different kind of issues (DB related, graph related).

These are related to nodes/edges and processing of the graph:

• Find out a testing strategy to track the nodes and edges - should we write tests with mock data or visualize through explorer UI
• Crawl should be guided by AI - AI guidance in crawler and scraper #203
• What parts of our engine take a lot of CPU time - CPU choking on multiple cores #185
• Evaluate stop conditions for engine - Evaluate stop conditions for engine #204
• Evaluate (automatic) restart conditions for engine - Evaluate (automatic) restart conditions for engine #207