Add support for multiple database connections with descriptions

[andre-c-andersen]

When adding multiple database connections, I noticed the postgres-mcp context tokens were duplicated:

To limit token usage for multi-connection scenarios, I added support for multiple connections:

• DATABASE_URI works as before as a default: So we have backwards compatibility.
• Add DATABASE_URI_* env vars for multiple connections (e.g., DATABASE_URI_APP → "app")
• Add DATABASE_DESC_* env vars for connection descriptions visible to AI
• Update all tools to accept required conn_name parameter
• Add ConnectionRegistry to manage multiple connection pools
• Display available connections in server context
• Update tests for new connection architecture

Additional fixes:

• Remove unnecessary connection pool invalidation during query execution. The psycopg AsyncConnectionPool already handles connection recovery automatically, so manual invalidation was redundant and caused misleading "Connection not available" errors for SQL errors (missing columns, tables, etc.)

Future work:

• Selectively choose which connections should be restricted. I didn't find a good design for this, so currently it applies globally to all connections. It also requires a deeper security audit than I have time for right now.