Manage backend accounts in postgres

[malteish]

#907 revealed that redis is only ever used for Session creation in the backend, where Session actually refers to an account. As the prisma model already defines an Account, we decided to remove redis and use the existing prisma Account model instead.

These fields exist in Session, if they are not crossed out they should be migrated to Account:

• [ ] ~~account: string;~~ -> id
• [ ] ~~signedUserProfile: SignedUserProfile;~~ -> must be read from resolver
• [ ] ~~token: string;~~ -> deprecated since jwt
• [ ] ~~publicMessageHeadUri?: string;~~ -> not used anywhere
• [ ] createdAt: number; -> keeping this for future reference
• [ ] ~~socketId?: string;~~ -> deprecated since socket manager
• [ ] ~~challenge?: string;~~ deprecated since jwt
• [ ] ~~profileExtension: ProfileExtension;~~ -> not used in backend
• [ ] ~~spamFilterRules?: unknown;~~ -> spam filter does not exist yet, add field if necessary