Harmonize delphi env variable names with the rest of the codebase

[jucor]

Problem

The rest of the polis codebase uses POSTGRES_* env variable names (POSTGRES_HOST, POSTGRES_DB, POSTGRES_USER, POSTGRES_PASSWORD, POSTGRES_PORT), defined in example.env and consumed by bin/ scripts, docker-compose.yml, CI workflows, and CDK.

Delphi's Python code uses a different set of names for the same settings:

Setting	Repo standard	Delphi
Host	POSTGRES_HOST	DATABASE_HOST
Port	POSTGRES_PORT	DATABASE_PORT
Database	POSTGRES_DB	DATABASE_NAME
User	POSTGRES_USER	DATABASE_USER
Password	POSTGRES_PASSWORD	DATABASE_PASSWORD
Connection string	DATABASE_URL	DATABASE_URL (same)

This causes confusion and bugs:

• docker-compose.yml bridges the gap with DATABASE_HOST=${POSTGRES_HOST}, which is fragile
• Test files that use the wrong set of names get wrong defaults and fail silently
• Two naming conventions for the same thing is just dirty

Files to update

Delphi files using DATABASE_HOST / DATABASE_* (non-standard names):

• delphi/polismath/database/postgres.py — PostgresConfig class and from_env()
• delphi/umap_narrative/polismath_commentgraph/utils/storage.py

Proposed fix

Rename DATABASE_HOST → POSTGRES_HOST, DATABASE_NAME → POSTGRES_DB, etc. in the delphi Python code to match the repo-wide convention. DATABASE_URL stays as-is (it's already consistent). The docker-compose.yml mapping (DATABASE_HOST=${POSTGRES_HOST}) can then be removed.

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