Fix: propagate sslmode env var

[ak7991]

Linked Issue

Relates to #5303

Description

Simply propagate the POSTGRES_SSLMODE environment variable to the self-hosted server's database configuration.

Specifically, this PR:

1. Updates server-side database connection URL builder (server/db.py) to append the sslmode queryparam when POSTGRES_SSLMODE is provided.
2. Propagates POSTGRES_SSLMODE into the default pgvector store configuration in server/main.py.

This is a focused follow-up to PR #5308 to address the server-side SSL.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactor (no functional changes)
• Documentation update

Test Coverage

• I added/updated unit tests
• I added/updated integration tests
• I tested manually (describe below)
• No tests needed (explain why)

Checked that the generated database connection string correctly appends ?sslmode=require (or other specified modes) when the POSTGRES_SSLMODE environment variable is set, allowing connections to PostgreSQL instances requiring SSL (e.g., hosted/managed Postgres services).

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have added tests that prove my fix/feature works
• New and existing tests pass locally
• I have updated documentation if needed

[ak7991]

Added the propagation of env var as requested.

Hey @kartik-mem0, would recommend to add support for directly providing db connection strings. sslmode support we added, but there are many other queryparams.
Example: postgresql://user:pass@primary-db:5432,replica-db:5432/mem0_db?target_session_attrs=read-write&connect_timeout=5.