Unify PostgreDB per Network for Proposal Discussion Forum and Budget Proposals

### Description
Currently, each GovTool environment (dev, qa, preview, preprod, mainnet) has its own local PostgreDB for offchain features such as the Proposal Discussion Forum and Budget Proposals. This segmentation complicates collaboration and external contributions.

This task aims to consolidate these databases into a single PostgreDB per Cardano network:
- One shared instance for all Preview-connected environments (dev, qa, preview)
- One instance for Preprod
- One instance for Mainnet

This change will simplify infrastructure, reduce data fragmentation, and enable easier external access and testing.

### Acceptance Criteria
- Dev, QA, and Preview environments share a unified PostgreDB instance for the Cardano Preview network.
- Preprod and Mainnet environments each retain their own dedicated PostgreDB.
- Infrastructure is updated to reflect the new database mappings.
- Proposal Discussion Forum and Budget Proposals function correctly in all environments after changes.